Sample Size Lift Calculator

Estimate sample size needed to detect a lift in conversion or performance metrics.

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%
days

Quick Facts

Lift
Smaller = Bigger Sample
Small lifts need more data
Power
Detect True Effects
Higher power means larger sample
Split
Traffic Matters
Uneven splits increase total sample
Decision Metric
Sample Size
Plan traffic requirements early

Your Results

Calculated
Sample Size
-
Per variant sample size
Total Sample
-
Total sample across variants
Target Rate
-
Target conversion rate
Daily Traffic
-
Daily traffic needed

Clear Lift Plan

Your defaults show a reasonable sample size for detecting lift.

Key Takeaways

  • This tool is built for scenario planning, not one-time guessing.
  • Use real baseline inputs before testing optimization scenarios.
  • Interpret outputs together to make stronger decisions.
  • Recalculate after meaningful context changes.
  • Consistency and execution quality usually beat aggressive one-off plans.

What This Calculator Measures

Estimate sample size needed to detect a lift between baseline and target conversion rates.

By combining practical inputs into a structured model, this calculator helps you move from vague estimation to clear planning actions you can execute consistently.

This calculator estimates sample size needed to detect a conversion lift with chosen confidence and power.

How the Calculator Works

n = (zα + zβ)^2 × p(1-p) / Δ^2
Target rate: baseline × (1 + lift).
Total sample: per-variant × 2.
Daily traffic: total ÷ duration.

Worked Example

  • 3.5% baseline with 15% lift targets 4.0%.
  • Confidence and power drive sample size.
  • Daily traffic estimate sets feasibility.

How to Interpret Your Results

Result BandTypical MeaningRecommended Action
Under 5kSmall test.Feasible quickly.
5k–20kModerate test.Plan for traffic needs.
20k–50kLarge test.Ensure enough duration.
50k+Very large.Consider bigger lift or longer test.

How to Use This Well

  1. Enter baseline rate and target lift.
  2. Select confidence and power.
  3. Choose traffic split.
  4. Set desired test duration.
  5. Review sample size and daily traffic.

Optimization Playbook

  • Target larger lifts: reduces required sample.
  • Adjust power: 80% reduces sample size.
  • Extend duration: lowers daily traffic needed.
  • Keep splits even: minimizes total sample.

Scenario Planning Playbook

  • Baseline: current rate and lift.
  • Bigger lift: raise lift to 20%.
  • Lower power: drop to 80%.
  • Decision rule: keep daily traffic achievable.

Common Mistakes to Avoid

  • Overestimating lift.
  • Ignoring power requirements.
  • Setting unrealistic test duration.
  • Using uneven traffic splits without planning.

Implementation Checklist

  1. Confirm baseline conversion rate.
  2. Choose realistic lift target.
  3. Set confidence and power.
  4. Plan traffic needs.

Measurement Notes

Treat this calculator as a directional planning instrument. Output quality improves when your inputs are anchored to recent real data instead of one-off assumptions.

Run multiple scenarios, document what changed, and keep the decision tied to trends, not a single result snapshot.

FAQ

Why does lift affect sample size?

Smaller lifts require more data to detect.

What power should I use?

80% is common, 90% for more confidence.

Can I use uneven splits?

Yes, but it increases total sample requirements.

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Frequently Asked Questions

How accurate are the results?
The Sample Size Lift applies a standard formula to your inputs — accuracy depends on how precisely you measure those inputs. For planning and estimation, results are reliable. For high-stakes or professional decisions, cross-check the output with a domain expert or primary source.
What sample size do I need for reliable results?
It depends on the desired confidence level, margin of error, and population variance. For a typical survey (95% confidence, ±5% margin), n ≈ 385 for a large population. Smaller samples are fine for exploratory analysis, but don't over-interpret the results — widen your confidence intervals to reflect the uncertainty.
How should I interpret the Sample Size Lift output?
The result is a calculated estimate based on the formula and your inputs. Compare it against the reference values or benchmarks shown on this page to understand whether your result is high, low, or typical. For decisions with real consequences, use the output as one data point alongside direct measurement and professional advice.
When should I use a different approach?
Use this calculator for quick, formula-based estimates. If your situation involves multiple interacting variables, time-varying inputs, or safety-critical decisions, consider a dedicated software tool, professional consultation, or direct measurement. Calculators are most reliable within their stated assumptions — check that your scenario matches those assumptions before relying on the output.