When should you cut a subreddit?

Cut a subreddit once it has spent about 2.3 times your target cost per conversion with zero conversions. With one conversion, wait until 3.9 times; with two, 5.3 times. Before that point, a bad-looking number is usually noise.

How much to spend before you judge

Multiply your target cost per conversion by the number in the right column. If a subreddit reaches that spend with no more conversions than the left column, cut it.

Where the 2.3 comes from

Suppose a subreddit converts at exactly your target. Then every time it spends one target's worth of money, you expect one conversion. Conversions arrive randomly, so the count over a stretch of spend follows a Poisson distribution with mean equal to spend divided by target.

P(0 conversions) = e^{−spend / target}  ≤ 0.10  ⇔  spend ≥ ln(10) × target ≈ 2.303 × target

So a subreddit that is truly on target shows zero conversions over 2.3 targets of spend less than 10% of the time. If you see that, it's probably not on target. The other rows solve the same equation for one, two, three conversions and so on. Your cost per click doesn't appear anywhere, which is why the rule works in any niche.

"Spend more" uses the mirror image: a subreddit is clearly beating target when a subreddit only at target would produce that many conversions less than 10% of the time.

What this calculator doesn't know

• Your other subreddits. It judges one subreddit alone. The full audit also uses how your whole account converts, so it can often call a subreddit sooner. In the sample report, r/webdev gets cut after $140 with no conversions, below the $184 that the 2.3× rule needs.
• Delayed conversions. If people usually buy days after clicking, recent spend hasn't had its chance yet. Leave out the last few days, or wait.
• View-through conversions. Reddit can count conversions from people who saw but didn't click an ad. Decide which conversions you trust before using any rule.