Pregnancy Calculator Online — Free Pregnancy Calculator Tool
CodexFuse Team · 2026-11-04T03:04:29.848Z · 8 min read min read
Use the free pregnancy calculator to estimate your due date, current week and trimester from your last period date and cycle length.
A pregnancy calculator is a free online tool that estimates your due date, current pregnancy week and trimester from two inputs: the first day of your last menstrual period (LMP) and your average cycle length. It does not diagnose pregnancy or replace medical care. It simply turns dates you already know into a clear timeline you can plan around.
How does a pregnancy calculator work?
A pregnancy calculator works by adding 280 days, or 40 weeks, to the first day of your last menstrual period, then adjusting for your cycle length. This is Naegele's rule, the standard obstetric method. If your cycle is longer than 28 days, the due date moves later by the difference; if it is shorter, the date moves earlier.
The pregnancy calculator on CodexFuse follows exactly this method. You enter your LMP and your average cycle length, and it returns your estimated due date, your current week and day, and your trimester. It also lets you copy the result in one click, which is handy when a clinic form or a partner asks for the date.
How to use the pregnancy calculator
Using the tool takes three steps, and nothing more complicated than a calendar date:
- Enter your LMP: type the first day of your last period, not the day it ended and not the day you tested positive.
- Enter your cycle length: type your average cycle in days. The tool accepts cycles from 20 to 45 days.
- Click Calculate Due Date: read your due date, current week and trimester, then copy the result if you need it elsewhere.
A worked example: from LMP to due date
Consider a clearly fictional example, so you can check the maths yourself. Sara's last period began on 15 January 2026, and her average cycle is 28 days. She opens the pregnancy calculator, enters 15 January 2026 as her LMP, leaves the cycle length at 28 days, and clicks Calculate Due Date.
The result is straightforward. Adding 280 days to 15 January 2026 gives a due date of 22 October 2026. On that date she would be exactly 40 weeks and 0 days pregnant. If she checked the tool on, say, 12 March 2026, it would show her at around 8 weeks, which places her in the first trimester.
Now change one input. If Sara's cycle were 32 days instead of 28, the calculation adds the 4-day difference, and her due date moves to 26 October 2026. If her cycle were 21 days, the date pulls back 7 days. That single adjustment is the difference between a useful estimate and a date that quietly misleads every plan built on it.
Save the copied due date in your phone calendar with a note of your LMP and cycle length. When a clinician asks how the date was worked out, you can answer in one sentence instead of reconstructing it from memory.
What if my cycle is not 28 days?
If your cycle is not 28 days, a pregnancy calculator adjusts your due date by the difference between your cycle and 28 days. A 32-day cycle pushes the due date 4 days later, a 24-day cycle pulls it 4 days earlier, and the CodexFuse tool applies this correction automatically for cycles between 20 and 45 days.
| Cycle length | Adjustment | Example: LMP 15 January 2026 |
|---|---|---|
| 21 days | 7 days earlier | Due date moves about a week earlier |
| 28 days | No adjustment | 22 October 2026 |
| 32 days | 4 days later | 26 October 2026 |
| 35 days | 7 days later | 29 October 2026 |
Which trimester am I in?
Your trimester follows directly from your pregnancy week. Weeks 0 to 12 are the first trimester, weeks 13 to 27 are the second trimester, and week 28 onward is the third trimester. The pregnancy calculator shows your trimester next to your week, so you do not need to memorise the boundaries.
For related date maths, the period calculator can help you pin down your cycle pattern, and the conception calculator explains the fertile window behind those dates. Once your baby arrives, the age calculator does the same kind of simple date arithmetic for months and years.
The free pregnancy calculator turns your last period date into a due date, week and trimester in seconds, so appointments, leave and preparations stop revolving around guesswork. Try it, copy your result, and keep the rest of your digital life just as simple with CodexFuse.
Get Started with CodexFuseHow accurate is a pregnancy calculator?
A pregnancy calculator gives an estimate, not a guaranteed birth date, because only around 5% of babies arrive on their exact due date. Most full-term births fall between 37 and 42 weeks, and a first-trimester ultrasound is the most accurate dating method because it measures the baby directly rather than relying on recalled dates.
This tool gives educational estimates only. It is not medical advice, it cannot confirm a pregnancy, and it cannot tell you whether a pregnancy is healthy. If you have pain, bleeding, or any worrying symptom, contact your healthcare provider promptly rather than recalculating.
What a pregnancy calculator does NOT do
Honest limits matter here, because a due date feels so precise that it is easy to over-trust it. The CodexFuse pregnancy calculator does not do the following:
- No conception-date input: this tool takes your last period date only. If you know your conception date, a separate due date calculator that accepts it will give a different estimate (conception plus 266 days).
- No fertility prediction: it estimates a due date from a past period; it does not predict ovulation or chances of conception.
- No medical screening: it cannot assess risk, interpret symptoms, or replace blood tests, scans or clinical advice.
- No stored history: the calculation runs in your browser, so your dates are not uploaded or saved for a later visit. Copy the result if you want to keep it.
Planning around your due date
Once you have a due date, use it as a planning anchor, not a deadline. A short checklist keeps the next steps manageable:
- Book the first appointment: call your clinic or midwife service early, and take your LMP, cycle length and calculator result with you.
- Work backwards from the date: plan leave, childcare research and major purchases around a window of a few weeks, not a single day.
- Track milestones weekly: note your week and trimester each week so screening windows and appointment topics are easier to follow.
- Keep your records together: one note with your LMP, due date, scan dates and questions saves repeated explanations at every visit.
Pin down your cycle first with the period calculator, then return to the pregnancy calculator with a more accurate cycle length for a better due-date estimate.
Frequently Asked Questions
How is a due date calculated?
By Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period, then adjust by your cycle length minus 28 days. With a 28-day cycle and an LMP of 15 January 2026, the due date is 22 October 2026.
What if I do not know my last period date?
Estimate as closely as you can, then confirm the date with a first-trimester ultrasound, which is the most accurate dating method. If your cycles are irregular, tell your clinician, because the calculator's estimate will be less reliable.
Can a pregnancy calculator confirm I am pregnant?
No. Only a pregnancy test and clinical confirmation can do that. A calculator assumes a pregnancy and estimates dates from the LMP you enter; it cannot detect pregnancy or assess its health.
Does a longer cycle mean a later due date?
Yes, typically. Each extra day of cycle length beyond 28 days pushes the due date about one day later, because ovulation usually happens later in a longer cycle. The tool applies this adjustment automatically.
Is the CodexFuse pregnancy calculator free?
Yes. It is free to use, runs in your browser, and includes a one-click copy button for your result. Your dates are not uploaded or stored by the tool.
When should I see a healthcare provider?
As soon as you have a positive test, or sooner if you have pain, bleeding or worrying symptoms. Take your LMP, cycle length and calculator result to the first appointment to help your clinician date the pregnancy accurately.
Try it, then make the rest of your setup just as simple
Open the pregnancy calculator, enter your last period date and cycle length, and copy your due date, week and trimester. Take that result to your first appointment and plan around a window of weeks, not a single day.
And if you run a clinic, practice, blog or small business, do not let the technology around it eat the time you just saved. Move your website, business email and hosting to CodexFuse today: one fully managed platform, on pay-as-you-go credits with no subscriptions and no server babysitting, so you can focus on the people you serve instead of the systems behind them. Get started with CodexFuse now and set up your managed setup in minutes.
