What Is an Amp? Amps, Volts, Watts, and Ohms in Plain English

CodexFuse Team · 2026-11-06T07:35:04.188Z · 18 min read min read

What Is an Amp? Amps, Volts, Watts, and Ohms in Plain English

If you are wondering what is an amp, the short answer is that an amp is the unit that measures electric current: how much electric charge flows past a point in a circuit every second. One ampere means one coulomb of charge passing a point each second, and its symbol is A. Amps tell you how much electricity is moving, while volts tell you how hard it is being pushed, watts tell you how much power that movement delivers, and ohms tell you how much the circuit resists the flow.

Beginners mix these four units up constantly, and the confusion costs real money and real safety: people buy the wrong charger, overload a circuit, or stare at a device label that lists watts when they needed amps. This guide sorts all four out in plain English, with the water-pipe analogy that every good beginner source uses, the two formulas that connect them, and worked examples for the devices in your home.

When a label gives you watts instead of amps, a watts to amps converter turns one into the other in a second. And if the question behind your search is how long a battery lasts at a given draw, our battery life guide explains that next step. Everything below was checked against the sources listed at the end on 6 November 2026.

The Four Units at a Glance

Four units do almost all the work in beginner electronics, and mixing them up is the most common beginner mistake. Each one measures a different thing, and the table below keeps them straight.

What Is an Amp?

An amp, short for ampere, is the SI unit of electric current. It measures the rate at which electric charge flows through a conductor, and one ampere equals one coulomb of charge passing a single point every second. The unit is named after André-Marie Ampère, the French physicist who helped found the study of electromagnetism, and it is one of the seven SI base units.

Put plainly, amps answer the question of how much electricity is flowing right now. A wire carrying 5 amps is moving five coulombs of charge past every point in the wire each second. That flow is what powers your devices, and it is also what your breakers and wires have to handle safely.

The Water-Pipe Analogy That Makes Amps Click

The water picture is a teaching aid, not a literal description of what happens inside a wire, but it is the analogy every good beginner source uses, from SparkFun to the US Department of Energy. Picture a garden hose connected to a tap. The water pressure in the hose is the voltage, the amount of water flowing through the hose each second is the current in amps, and a kink or a narrow nozzle is the resistance in ohms.

Open the tap wider and the pressure rises, so more water flows. Narrow the hose and the same pressure pushes less water through. The power the water can deliver, say to spin a small waterwheel, depends on both the pressure and the flow together, which is why watts are volts times amps.

Keep the mapping on one line and it never confuses you again: volts push, amps flow, ohms resist, watts work. The rest of this guide is just that idea with numbers attached.

What Does Amp Mean in Electricity?

The ampere definition answers what is an ampere in formal terms: one ampere is one coulomb of electric charge passing a point in a circuit per second. A coulomb is itself a unit of charge, so an amp is really a rate, charge per second, in the same way miles per hour is a rate. That is why current is written as I in formulas, from the French intensité de courant.

Since 20 May 2019, the ampere has been defined by fixing the elementary charge at exactly 1.602176634 × 10^-19 coulombs. In plain terms, one amp means about 6.241509074 × 10^18 electrons passing a point every second. Before 2019 the definition used the force between two parallel wires, but the modern definition counts charge directly.

You will also meet the word amperage, which simply means current measured in amps. A device that draws 2 amps is like a tap running at 2 litres a second: both describe a flow rate, not a total amount.

Amps vs Volts: Flow or Pressure?

Amps vs volts is the first comparison to get right, because the two are easy to swap in conversation. Volts measure the electrical pressure that pushes charge along, while amps measure how much charge actually flows. A 120-volt US wall outlet pushes with the same pressure whether you plug in a phone charger or a hair dryer, and what changes is the current each device draws.

Think of two hoses at the same pressure: a wide one carries far more water per second than a narrow one. The pressure, in volts, is the same, but the flow, in amps, differs because the resistance differs. That is exactly what happens when two devices share the same 120-volt supply and draw different currents.

High volts with almost no amps can still be dangerous, since static electricity is thousands of volts at tiny currents, and high amps at low volts can melt wires. Neither number alone tells the whole story, which is why both appear on every device label.

Amps vs Watts: Why Amps Are Not Power

An amp is not a unit of power, and that is the single most useful sentence in this guide. Power is measured in watts, and watts come from volts times amps together, P = V × I. Amps alone say how much charge flows, while watts say how much work that flow can do.

A phone charger at 5 volts and 2 amps delivers 10 watts, while a laptop charger at 19 volts and about 3.42 amps delivers about 65 watts. The laptop charger moves more charge per second, but it also pushes harder, so the power is more than six times greater even though the current is less than twice as large.

One more mix-up to clear up while we are here: watts measure power, and watt-hours measure energy. A 60-watt bulb uses 60 watts while it is on, but run it for one hour and it has used 60 watt-hours of energy, or 0.06 kilowatt-hours, the unit on your electricity bill. Power is the rate, and energy is the rate times the time.

Volts vs Watts vs Amps vs Ohms: The Full Comparison

Put all four together and each unit has exactly one job. Volts push the charge, amps count the flow, ohms slow it down, and watts measure the work done. Change any one and the others shift with it, because the two formulas tie them together.

Raise the volts and keep the resistance fixed, and the amps rise in step, which is Ohm's law at work. Raise the resistance and keep the volts fixed, and the amps fall. Raise either the volts or the amps and the watts rise, because watts are the product of the two.

A simple way to remember it is to read a device label backwards: a 1,500-watt heater on a 120-volt supply must draw 12.5 amps, because 1,500 divided by 120 is 12.5. The label gives you two numbers, and the formulas give you the third.

Ohm's Law in Plain English

Ohm's law states that the current through a conductor equals the voltage across it divided by its resistance, I = V ÷ R. Georg Ohm published the relationship in 1827, and it is the single most-used formula in beginner electronics. Rearranged, it also reads V = I × R and R = V ÷ I, so any one of the three can be found from the other two.

Both formulas rearrange into a small family, and the table below lists the whole set with one example each.

A 12-volt battery across a 6-ohm resistor pushes 2 amps through it, because 12 divided by 6 is 2. Double the resistance to 12 ohms and the same battery pushes only 1 amp. That is why a resistor is the standard tool for limiting current: it sets the flow for a given voltage.

Ohm's law does not hold for every material, since components such as diodes and filament bulbs are non-ohmic, and their resistance changes as they heat up. For ordinary resistors and most beginner circuits, though, the law is exact, and it is the reason an LED resistor calculator exists: a resistor drops the voltage to the level an LED can take safely.

The Watts Formula and the 5-Step Amp Check

The watts formula is the one you will use most: P = V × I, power equals volts times current. Turned around, it gives current from power and voltage, I = P ÷ V. That single rearrangement answers almost every how-many-amps question on a device label.

Here is the whole check in five steps, and it works for any plug-in device.

1. Read the watts (W) off the device label or manual.
2. Read the volts (V): 120 V for a US wall outlet, or the charger output volts, often 5 V, 9 V, 12 V, 19 V or 20 V.
3. Divide watts by volts: amps = watts ÷ volts.
4. Compare the answer with the circuit: a US 15-amp circuit allows 1,800 watts at most, and 1,440 watts for a continuous load.
5. If the amps are above the circuit or the charger rating, move the device to another circuit or pick a bigger supply.

The US Department of Energy's own worked example follows the same steps: an appliance rated 3 amps multiplied by 120 volts is 360 watts. Run the division the other way and 360 watts divided by 120 volts is 3 amps again.

How Many Amps Is a Watt?

How many amps is a watt depends entirely on the voltage, because one watt is one volt times one amp. At 5 volts, one watt is 0.2 amps, at 12 volts it is about 0.083 amps, and at 120 volts it is about 0.0083 amps. The table below works it out for the wattages people actually ask about.

For exact values at any wattage, the watts to amps conversion chart on this site lists the common combinations in one place. Every number in this article was computed in Python, and the code below reproduces them all.

print("120 V x 5 A =", 120 * 5, "W")  # 600 W
print("600 W / 120 V =", 600 / 120, "A")  # 5.0 A
print("1500 W / 120 V =", 1500 / 120, "A")  # 12.5 A
print("10 W / 5 V =", 10 / 5, "A")  # 2.0 A
print("65 W / 19 V =", round(65 / 19, 2), "A")  # 3.42 A
print("1200 W / 120 V =", 1200 / 120, "A")  # 10.0 A
print("20 A x 120 V =", 20 * 120, "W")  # 2400 W
print("15 A x 120 V =", 15 * 120, "W")  # 1800 W
print("100 W / 20 V =", 100 / 20, "A")  # 5.0 A
print("100 W / 120 V =", round(100 / 120, 2), "A")  # 0.83 A
print("100 W / 12 V =", round(100 / 12, 2), "A")  # 8.33 A
print("1875 W / 120 V =", round(1875 / 120, 2), "A")  # 15.62 A
print("10 W / 120 V =", round(10 / 120, 3), "A")  # 0.083 A
print("80% of a 15 A circuit =", 120 * 12, "W")  # 1440 W
print("80% of a 20 A circuit =", 120 * 16, "W")  # 1920 W
print("100 Wh at 10 W =", 100 / 10, "hours")  # 10.0 hours
print("1 A in electrons per second =", 1 / 1.602176634e-19)  # 6.241509074460763e+18
# Expected output:
# 120 V x 5 A = 600 W
# 600 W / 120 V = 5.0 A
# 1500 W / 120 V = 12.5 A
# 10 W / 5 V = 2.0 A
# 65 W / 19 V = 3.42 A
# 1200 W / 120 V = 10.0 A
# 20 A x 120 V = 2400 W
# 15 A x 120 V = 1800 W
# 100 W / 20 V = 5.0 A
# 100 W / 120 V = 0.83 A
# 100 W / 12 V = 8.33 A
# 1875 W / 120 V = 15.62 A
# 10 W / 120 V = 0.083 A
# 80% of a 15 A circuit = 1440 W
# 80% of a 20 A circuit = 1920 W
# 100 Wh at 10 W = 10.0 hours
# 1 A in electrons per second = 6.241509074460763e+18

What Everyday Devices Actually Draw

Nameplate wattages vary by model, so treat the numbers below as typical ranges from the sources listed at the end, not as exact figures for every device. The pattern to notice is how quickly the amps climb as the watts rise on the same 120-volt supply.

65 W is a common laptop charger rating, not a rule for every laptop: thin ultrabooks often ship with 30–45 W chargers, while gaming laptops go far higher, and a 65 W charger is a ceiling, not a constant draw. And 1,875 W is the top of the range for a US hair dryer, not what every dryer draws, since most sit between 1,200 and 1,875 watts.

The battery side of the same question is how long a charge lasts, and that is a watts-and-hours question rather than an amps question. A 100-watt-hour battery runs a 10-watt load for 10 hours, a 100-watt load for 1 hour, and a 5-watt load for 20 hours, because hours equal watt-hours divided by watts. Our battery life calculator works the same sum out for any battery and load.

Why Breakers Trip: Amps, Heat, and the 80% Rule

A breaker that trips is doing its job: it is cutting the current before the wires behind your walls overheat. US homes run on about 120 volts at 60 hertz, and most general-purpose circuits are protected by 15-amp or 20-amp breakers. Multiply the two and a 15-amp circuit allows 1,800 watts at most, while a 20-amp circuit allows 2,400 watts.

The 80% rule is the part people miss: a circuit should carry no more than 80% of its rating for a continuous load, so a 15-amp circuit is really a 1,440-watt circuit for anything that runs for hours. That is why a 1,500-watt space heater plus a 1,200-watt microwave, 2,700 watts together, trips even a 20-amp breaker, whose ceiling is 2,400 watts.

The heat is the reason: a wire's heating grows with the square of the current, P = I²R, so a small rise in amps makes a much bigger rise in heat. Inside a thermal-magnetic breaker, sustained overcurrent heats a bimetal strip until it bends and releases the latch, cutting the power. If a breaker keeps tripping, the safe response is to move devices to another circuit, because the breaker is telling you that circuit is overloaded.

CodexFuse does not offer a standalone amp meter tool: the electrical tools on this site are the watts-to-amps converter and the LED resistor calculator linked above, plus the battery calculators for the runtime side of the question.

Sources

• Wikipedia, "Ampere" — https://en.wikipedia.org/wiki/Ampere — definition of the ampere, the 2019 SI redefinition, and the elementary charge value
• Wikipedia, "Ohm's law" — https://en.wikipedia.org/wiki/Ohm%27s_law — V = I × R, the 1827 publication date, and the hydraulic analogy
• Wikipedia, "Electric power" — https://en.wikipedia.org/wiki/Electric_power — P = V × I, P = I²R, and the watt as one joule per second
• US Department of Energy, "United States Electricity Industry Primer" (July 2015) — https://www.energy.gov/sites/prod/files/2015/12/f28/united-states-electricity-industry-primer.pdf — P = IV and the waterwheel analogy
• US Department of Energy, "Energy Efficiency at Home" lesson — https://www1.eere.energy.gov/education/pdfs/lesson301.pdf — the worked example of 3 amps × 120 volts = 360 watts
• US Department of Energy, "Electric Meters" (archived) — https://webarchive.library.unt.edu/web/20170127061121mp_/https://energy.gov/energysaver/electric-meters — watts vs watt-hours and the kilowatt-hour on bills
• EcoFlow, "How Many Watts Can an Outlet Handle? 15A vs 20A Explained" — https://www.ecoflow.com/us/blog/how-many-watts-can-an-outlet-handle — US 120 V and 60 Hz, 15 A and 20 A circuits, the 80% rule
• EngineerFix, "What Is the Standard Wall Outlet Voltage and Amps?" — https://engineerfix.com/what-is-the-standard-wall-outlet-voltage-and-amps/ — 120 V nominal, 60 Hz, 15 A or 20 A residential circuits
• SparkFun Education, "Voltage, Current, Resistance" — https://sparkfuneducation.com/concept-library/voltage-current-resistance.html — the water-tank analogy and the 1 volt, 1 amp, 1 ohm relationship
• University of Maryland Makeability Lab, "L1: Voltage, Current, and Resistance" — https://makeabilitylab.github.io/physcomp/electronics/electricity-basics.html — voltage as water pressure, current as flow rate
• Virginia Tech Extension, "Estimating Appliance and Home Electronic Energy Use" — https://www.pubs.ext.vt.edu/content/pubs_ext_vt_edu/en/2901/2901-9014/2901-9014.html — microwave 750–1,100 W, hair dryer 1,200–1,875 W, portable heater 750–1,500 W
• Enphase, "How many watts do common home appliances use?" — https://enphase.com/blog/homeowners/how-many-watts-do-common-home-appliances-use — microwave 600–1,200 W, space heater 750–1,500 W, 1,500 W for 3 hours = 4.5 kWh
• ComputerKnown, "How Many Watts Is a Laptop Charger?" — https://computerknown.com/how-many-watts-is-a-laptop-charger/ — 19 V × 3.42 A ≈ 65 W, mainstream laptops 45–65 W
• ASUS 10W Adapter product page — https://www.asus.com/eg-en/accessories/adapters-and-chargers/asus-adapters-and-chargers/asus-10w-adapter-micro-usb-cable/ — 10 W output at 5 V and 2 A
• BytePreview, "USB Power Delivery" — https://www.bytepreview.com/en/articulos/usb-power-delivery-the-protocol-that-negotiates-fast-charging — 100 W at 20 V × 5 A, 240 W extended range
• Home Depot, EcoSmart 60 W-equivalent LED — https://www.homedepot.com/p/reviews/EcoSmart-60-Watt-Equivalent-A19-Dimmable-Energy-Star-LED-Light-Bulb-Bright-White-4-Pack-A7A19A60WESD02/302555863/1 — 8.5 W replacing a 60 W bulb
• NRDC light-bulb FAQ — https://oldsite.pandle.co.uk/in-the-dark-about-picking-a-light-bulb-this-faq-can-help-00212179.html — an LED delivering 800 lumens with as little as 10 watts
• McGraw-Hill Access Engineering, "Circuit Protection" — https://www.accessengineeringlibrary.com/index.php/binary/mheaeworks/0b94971af11c5e96/fb8fcdf364650758d17b081407c973d9c7b2a8d50b7739214adafdf7192c374f/circuit-protection.pdf — breakers protecting wires from overheating, the bimetal strip releasing the latch
• Right Connections Electric, "Circuit breakers and fuses" — https://www.rightconnectionselectric.com/electrical-safety/circuit-breakers-and-fuses — breakers protecting wires from overloads that overheat and melt insulation
• Green Building Advisor, "What's the Difference Between Energy and Power?" — https://www.greenbuildingadvisor.com/article/whats-the-difference-between-energy-and-power — a 60-watt bulb for one hour uses 60 watt-hours, or 0.06 kilowatt-hours

Frequently Asked Questions

How many amps is 100 watts?

Divide watts by volts. On a 120-volt US supply, 100 watts is about 0.83 amps, because 100 divided by 120 is 0.83. On a 12-volt supply the same 100 watts is 8.33 amps, because the lower voltage needs more current to deliver the same power.

Is an amp the same as a watt?

No. An amp measures current, the flow of charge, while a watt measures power, the rate of doing work. Watts equal volts times amps, so you need both the volts and the amps to know the watts.

How many amps does a phone charger use?

A basic USB charger delivers 5 volts at 2 amps, which is 10 watts. A USB-C Power Delivery charger can reach 20 volts at 5 amps, or 100 watts, and the extended range reaches 240 watts.

What happens if the amps are too high?

The breaker trips, cutting the power before the wires overheat. A wire carrying more current than it is rated for heats up with the square of the current, so sustained overload can melt insulation and start a fire, which is exactly what the breaker prevents.

How do I calculate amps from watts?

Divide watts by volts: amps = watts ÷ volts. A 1,500-watt heater on a 120-volt outlet draws 12.5 amps, and a 600-watt appliance on the same outlet draws 5 amps.