Calculators
A/B Test Sample Size Calculator: Visitors per Variant
Sample Size Calculator — 100% free, unlimited, no sign-up cost.
Visitors per variant ≈ 16 × p(1−p) ÷ d², where p is your baseline conversion rate and d the smallest lift you want to detect. At 5% baseline and a 1-point lift that is 7,600 per variant.
Enter your current conversion rate and the smallest improvement worth detecting. The calculator returns an approximate number of visitors per variant, using a common rule of thumb for about 95% confidence and 80% power.
Use the Sample Size Calculator online at no cost.Common tasks this tool helps with
- sample size calculator formula
- how to calculate sample size
- free sample size calculator
- sample size calculator with example
Try Sample Size Calculator free
The Required Sample Size formula
n = (Z² × p(1−p)) ÷ E²
Z is the z-score for your confidence level (1.96 for 95%), p is your expected conversion rate, E is your acceptable margin of error.
Example
You expect a 10% conversion rate and want a 2% margin of error at 95% confidence.
(1.96² × 0.1 × 0.9) ÷ 0.02² = ≈ 865 visitors per variant
Worked example: 5% baseline, 1-point lift
Current conversion rate 5%. You want to detect a lift to 6%, so the minimum detectable effect is 1 percentage point.
- 16 × 0.05 × 0.95 ÷ 0.01² = 7,600 visitors per variant.
- Two variants therefore need about 15,200 visitors in total.
- Halving the detectable lift to 0.5 points roughly quadruples the sample to about 30,400 per variant.
What to do next: Divide the total by your weekly traffic to estimate duration, and run the test for whole weeks. This is an approximation; dedicated experiment tools use exact power calculations.
Why it's free
Tests that end too early produce false winners. Knowing the required sample before launch tells you whether a test is feasible with your traffic, and why detecting a small lift needs far more visitors than a large one.
What you get
- An approximate visitor count per variant
Who uses the Sample Size Calculator
- CRO teams planning tests
- Marketers checking if traffic is enough to test
- Founders deciding whether to A/B test at all
How it works
- Enter your baseline conversion rate as a percentage
- Enter the minimum detectable effect in percentage points
- Read the approximate visitors needed per variant
Frequently asked questions
Is the Sample Size Calculator free?
Yes. Every one of GrowthGadgetAi's 100+ Marketing Toolkit tools, including the Sample Size Calculator, is completely free with unlimited use — no credits, no plan requirement and no credit card.
Do I need to create an account to use it?
No. It runs on this page, in your browser, without signing in. Nothing you enter is sent to a server.
How is the Sample Size Calculator's result calculated?
It's a direct formula, not an AI guess — the Sample Size Calculator computes your visitors / variant from the numbers you enter, instantly and the same way every time.
How do I use the Sample Size Calculator?
1) Enter your Baseline conversion rate (%) and Minimum detectable effect (pts). 2) See your visitors / variant instantly, no submit button needed. 3) Use the number in your reporting or planning. The whole process takes about a minute and runs on this page, on desktop or mobile.
What is the formula behind the Sample Size Calculator?
Required Sample Size = n = (Z² × p(1−p)) ÷ E². Z is the z-score for your confidence level (1.96 for 95%), p is your expected conversion rate, E is your acceptable margin of error.
How accurate is the 16 × p(1−p) ÷ d² formula?
It is a widely used approximation for roughly 95% confidence and 80% power with a two-sided test. Exact planning tools can differ slightly, especially for very low or high baselines.
More free Calculators tools
Browse all 100+ free tools
Go further with AI Marketing Analytics & Reporting Tool: Move from campaign numbers to the next decision.
Read more: landing page optimization
Related tools