A/B Test Calculator

Sample Size & Duration

Before running an A/B test, you need to know two things: how many visitors each experience needs to collect, and how long that will take. This calculator gives you both — so you know exactly when it's safe to read the results.

New to A/B testing?

Let me walk you through it step by step

What are you measuring?

Best for goals measured by % of visitors who complete an action (click, sign-up, purchase).

Your current numbers

Current conversion rate

e.g. if 25 out of 1,000 visitors convert, enter 2.5. Find this in your analytics tool.

What's the smallest improvement worth acting on?

% lift

10% lift on 2.5% baseline → target rate 2.75%

Your traffic

Daily visitors to page being tested

% of traffic in the test

Not total site traffic — only visitors to the page being tested. Leave traffic % at 100 unless you're running a partial rollout.

How many experiences are you testing? (control + variants)

Planned start date (optional)

Results

Visitors needed per experience

64,215

128,430 total across all experiences

Don't read results until each experience reaches this number

Minimum run time

33 days

at 4,000 visitors/day to that page

Test duration healthLong (2–8 weeks)

1 day4 wks8 wks+

Tip: Consider rounding to 35 days to capture full week cycles — day-of-week traffic variation can skew results.

Reminder: Checking results before your planned end date inflates the false positive rate significantly. Trust the plan.

Executive Summary

Executive summary

Run this test for at least 35 days (rounded to full week cycles) before reading results. You need ~64,215 visitors per variant before the result is statistically trustworthy. At your current traffic of 4,000 visitors/day, that's roughly 5 weeks.

Test Reliability

Test reliability

4 / 5 bars — Strong statistical setup

MDE Sensitivity

MDE sensitivity

How duration changes if you adjust how big a lift you need to detect:

MDE	Sample/variant	Duration
5%	250,917	126d
10%← current	64,215	33d
15%	29,199	15d
20%	16,794	9d
25%	10,984	6d

Activity Note

AT activity note

// EL Calculator — 2026-05-09
// Metric:     Conversion Rate | Baseline: 2.5%
// MDE:        10% relative lift
// Sample:     64,215 per variant (128,430 total)
// Duration:   ~33 days at 4,000 visitors/day
// Confidence: 95% | Power: 80% | Variants: 2

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