When to use this formula
Use this formula when your inputs match the variables and units shown below. It is most useful for checking a calculator result, recreating the calculation in a spreadsheet or understanding which input has the biggest effect.
Formula
LMP method: estimated due date = last menstrual period + 280 days + cycle adjustment
Variables
The cycle adjustment compares the entered cycle length with a 28-day reference cycle. Conception-date mode typically adds about 266 days.
Example
With a last menstrual period of January 1 and a 28-day cycle, the estimated due date is October 8 in a non-leap-year calculation context.
Assumptions and limitations
Dating can differ based on ultrasound, ovulation timing, cycle variability and clinician assessment. This is not medical advice.
When the formula is not enough
- If the result depends on live prices, rates or official thresholds, check the latest value from the named source before relying on it.
- If the topic is medical, tax, legal, lending or safety related, use the result as a learning aid and check primary guidance before acting.
- If units or time periods differ, convert them before comparing results.
- If rounding affects the decision, keep extra precision until the final step.
FAQ
Why look at the formula instead of only the answer?
The formula shows which inputs actually drive the result. That makes it easier to spot a wrong unit, compare two scenarios or explain the answer to someone else.
Can different calculators use different formulas for the same topic?
Yes. Some topics have multiple accepted methods or simplified variants. When that matters, the calculator should say which method it uses and what is excluded.
Are formula pages updated?
Stable math formulas need occasional review. Formulas that depend on changing rules, prices or thresholds need a dated source before the page can make stronger claims.