Free calculator

How many form submissions does your CAPTCHA cost?

Enter three numbers from your own site. The calculator shows the loss per month and per year and the formula behind it. Nothing is sent anywhere.

The short answer

A visible CAPTCHA costs you every submission from people who give up at the challenge. With visitors per month, completion rate and the share who give up, the loss is submissions today × share ÷ (1 − share). There is no reliable industry-wide figure for that share, so this calculator uses yours.

Your numbers

The pre-filled numbers are an example, not a finding. Replace them with your own.

Visitors to the form per month

People who see the form, not your whole site.

Completion rate today (%)

Share of those visitors who submit the form.

Share who give up because of the CAPTCHA (%)

Your own measurement. The section below explains how to get it.

Value per submission (optional)

In your own currency, for example the average lead or order value.

Your result

Submissions per month today: 300
Submissions per month without the hurdle: 333
Lost per month: 33.3
Lost per year: 400

The calculation

Three lines, so you can check the result and quote it:

Submissions today: A = visitors × completion rate
Submissions without the hurdle: A0 = A ÷ (1 − a)
Lost per month: A0 − A = A × a ÷ (1 − a)

a is the share of people who would submit but give up at the CAPTCHA, written as a fraction of 1 (10 % means a = 10 ÷ 100). The loss grows faster than a, because the share refers to everyone who would have submitted: at 50 %, as many submissions are lost as arrive today.

Why there is no default rate

Blog posts say a CAPTCHA costs up to 40 % of conversions and attribute this to a Stanford study. We could not find a primary source for that figure. The Stanford work we found measures how long people take to solve CAPTCHAs and how often they fail, not conversions. Until someone publishes a primary source, the only number that counts is the one you measure on your own form.

How to measure the share on your own form

1Pick one form and a period in which nothing else on the page changes.
2Record the visitors who reach the form and the genuine submissions while the visible CAPTCHA is on.
3Run a comparable period, or split the traffic, with the CAPTCHA replaced by an invisible check. Count genuine submissions only: spam will go up, and it must not inflate the result.
4Compute a = 1 − (submissions with the CAPTCHA ÷ submissions without it) and enter it above.

Sources

Bursztein, E., Bethard, S., Fabry, C., Mitchell, J. C., Jurafsky, D.: How Good Are Humans at Solving CAPTCHAs? A Large Scale Evaluation. IEEE Symposium on Security and Privacy, 2010. Measures solving time and accuracy of people, not conversion.

Questions about the calculator

How much conversion does a CAPTCHA cost?

There is no reliable industry-wide figure. The often-quoted 40 % has no primary source that we could find. Measure it on your own form: compare genuine submissions with and without the visible CAPTCHA, then enter the result here.

Why does the loss grow faster than the share who give up?

The share refers to everyone who would have submitted without the CAPTCHA, not to those who still arrive. If half of them give up, as many submissions are lost as you still receive.

Is my data sent anywhere?

No. The calculation runs in your browser. Nothing you type is transmitted or stored.

What counts as giving up because of the CAPTCHA?

People who start the form and leave at or after the challenge: because they failed it, hesitated, or could not use it. Visitors who never intended to submit do not count.

Fewer puzzles for your visitors?

SilentShield checks behaviour in the background instead of asking every visitor to solve a puzzle. See what the other providers load into your page, or try it free.

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