The 15% rule: how idle games pace your next purchase

Open almost any idle game and buy the same thing twice. The second one costs more. In Cookie Clicker every building costs 15% more than the copy before it, so the price of the next one is base × 1.15^owned. That single exponent does more pacing work than any other number in the game.

Income grows in a line. Prices grow in a curve.

Each copy you own adds the same amount of income, so income rises linearly. The price rises exponentially. Early on the line wins: you can afford something every few seconds and the game feels generous. Somewhere past a few dozen copies the curve overtakes it and waits stretch from seconds to minutes to hours. Drag the growth slider and watch where the bend moves.

What designers do at the bend

The stall is the point. Every escape from it is a decision the game hands back to you:

• A new generator. It starts cheap on its own curve, so buying it resets the feeling of speed.
• Milestone multipliers. Owning 25, 50 or 100 of something doubles its output, which turns “one more copy” into a target worth saving for.
• Upgrades that cross-link generators. They make the order you buy things in matter.
• A reset. When everything stalls at once, a prestige reset trades your progress for a permanent multiplier. See why resetting feels like progress.

Buying ten at once

Because prices form a geometric series, the cost of the next k copies has a closed form:price × (r^k − 1) / (r − 1), where r is the growth rate. That is how “buy ×10” and “buy max” buttons work without looping, and it is worth knowing: at r = 1.15, ten copies cost about 20 times the price of one.

Try it on a live reef

Tidekeeper uses this exact model. Open it from Play, buy plankton until the wait gets uncomfortable, then compare that moment with the lab above.