How a savings rate is measured

A savings rate does two things at once, which is unusual for a single ratio. It sets how quickly the pot grows, and it sets how large the pot has to be, since a household saving 20 percent is living on the other 80 percent.

That is why comparing rates is more informative than comparing amounts. $15,000 on $75,000 is 20 percent, and the same $15,000 on a much larger income is a much weaker position, because the spending it implies is higher and so is the retirement target it has to fund.

Divide by gross income and the rate is lower. Divide by take-home pay and it is higher, because tax has already been removed from the denominator. Neither is wrong, and quoting one while comparing it to the other is.

The practical rule is to pick a definition and keep it. Then, when comparing with anyone else's figure, ask which they used before drawing any conclusion. The savings rate calculator takes both numbers as inputs so the definition is yours and it is visible.

Money into an investment account clearly counts. So does a pension contribution, and usually so does the employer's contribution, provided it also appears in the income figure. Mortgage principal repayment is arguable: it builds equity, so many people include it, and many do not.

What should not count is money set aside for something you will spend this year, since that is timing rather than saving. Keeping the boundary consistent across years is what makes the trend readable, and the trend is the useful part.

Higher rates compound twice: more money invested, and a lower spending level to sustain later. That relationship is what drives the arithmetic behind how FIRE numbers work, and it is why a few points on the rate move a retirement date by years rather than months. Read this alongside how coast FIRE works and the FIRE number calculator, which turns spending into a target. This is educational material, not financial advice.