How Idle Game Math Works: Costs, Milestones and Multipliers

By Kamil Fiks · 16 October 2026 · 7 min read

Idle games look like they are about clicking, but they are really about arithmetic. Every “should I buy this now?” question has a numeric answer, and once you see the handful of formulas most idle games share, you can answer it in your head. This guide walks through those formulas with real numbers from the games on Idle Center. They are not made-up examples: they are the values the games actually use.

1. Costs grow by a percentage

Almost every idle game prices its buildings with a geometric cost curve: each copy costs a fixed percentage more than the one before. In Primordial Tap that percentage is 15%, so the price of an upgrade is its base cost × 1.15^owned.

Take the first upgrade in the game, Primordial Spark, with a base cost of 100 energy. The first copy costs 100, the tenth costs 351, and the first ten together cost about 2,030, roughly twenty times the price of the first. Keep buying and the curve gets steep: the fiftieth copy of any upgrade costs about 943 times its base price.

Two things follow from this. First, there is no point hoarding one building forever, because each copy is worse value than the last. Second, a new building with a higher base cost is often cheaper per unit of income than the twentieth copy of an old one. Comparing income gained per unit spent across everything you can buy is the most useful habit in the genre.

Buying in bulk

The cost of buying several copies at once is the sum of a geometric series: first price × (1.15ⁿ − 1) ÷ 0.15. You never need to compute this yourself, because the game shows the total, but it explains why a ×10 button costs far more than ten times the current price. Bulk buying is a convenience, not a discount.

2. Milestones change the maths

Many idle games reward owning a round number of something. These milestones break the “each copy is worse value” rule, and that makes them the best purchases in the game.

In Primordial Tap, an upgrade's whole output is multiplied by 2 at 10 copies, by 3 at 25 and by 5 at 50. Those multipliers replace each other, so 25 copies give ×3, not ×6. Here is Primordial Spark around its second milestone:

One purchase lifts the upgrade's output by 56%. No ordinary purchase comes close. In IdleCenter, the effect is even stronger, because staff milestones stack: every role doubles its flat output at 10, 25, 50 and 100 hires. Nine Developers give 9 build work per second, while ten give 20.

Rule of thumb: when something is a few copies short of a milestone, finish it before buying anything new.

3. Multipliers multiply

Income in idle games is usually a product of several factors. In IdleCenter, a released game's income is its base rate × quality × how well its topic and genre fit × launch fame × market hype × your research bonuses, then reduced by crowding and age. Because these multiply, a 20% boost to any one factor is a 20% boost to everything.

Multiplying also means that balanced boosts beat lopsided ones. Two factors at 1.5 give 2.25. One factor at 2.0 and one at 1.0 give only 2.0. When you can choose which part of your engine to improve, the weakest factor is often the best target.

Additive bonuses behave differently. If a game adds +3% income per point of something, as IdleCenter's Reputation does, 18 points give +54% (×1.54), not 1.03¹⁸ ≈ ×1.70. Read upgrade text carefully: “+X%” and “×X” are not the same thing.

4. Decay and half-life

Some idle games make income fade over time, which forces you to keep producing. IdleCenter models a game's sales with a half-life: income halves every two minutes, and the game retires after seven half-lives, about 14 minutes, when it is earning under 1% of its launch rate.

The useful number here is the total. A game that starts at r dollars per second earns about r × half-life ÷ ln 2 over its life. With a two-minute half-life, that is about 173 seconds of its launch income. So a release is worth roughly three minutes of its opening rate, and most of that arrives in the first few minutes. That is why a steady stream of releases beats an occasional big hit, and why research that stretches the half-life (such as Long Tail, +25%) raises the value of every future game by the same 25%.

5. Prestige and square roots

Prestige currencies are usually calculated with a diminishing formula, very often a square root. IdleCenter's Reputation is the square root of what your studio earned ÷ 15,000. The minimum Legacy, at $5,000,000 earned, gives 18 Reputation.

Doubling your earnings only multiplies Reputation by about 1.41. Because earnings grow quickly late in a run, it is still worth staying a while past the minimum. But the longer you wait, the less each extra minute adds. The prestige guide looks at this trade-off in each of our games.

6. Payback time: the one formula to remember

If you remember one thing, make it this: payback time = cost ÷ income gained. An upgrade that costs 1,000 and adds 10 per second pays for itself in 100 seconds. Compare the payback times of everything you can buy, and buy the shortest first. Milestones, cheap multipliers and new buildings usually win, while the umpteenth copy of an old building usually loses.

The reference pages for each game list the real numbers you need, such as base costs, caps and cost growth, so you can do this comparison yourself. Start with the Primordial Tap upgrade list, the IdleCenter staff and market numbers or the Realm Forge economy page.

More guides

• What Are Idle Games? Incremental and Clicker Games Explained

  What idle, incremental and clicker games are, where the genre came from, and how active, idle and offline play differ, with examples from our games.

• When to Prestige: Resets in Idle Games Explained

  How prestige resets work and when to take them, with the real rules of Extinction in Primordial Tap, Studio Legacy in IdleCenter and Realm Forge exile.

Ready to try it yourself? Pick a free game on the Idle Center home page.