Everyday Money Math: Rounding, Stacked Discounts, Loans and Inflation

We tested nine money calculators against exact arithmetic on tens of thousands of cases. These are the results that surprise people most.

Money Is Counted in Cents

Most money mistakes in software come from treating dollars as ordinary decimal numbers. Computers store 0.1 + 0.2 as 0.30000000000000004, and a price times a tax rate often lands exactly on half a cent. Our calculators keep money in whole cents and round half a cent up, the way a register does, and every one of them was checked against exact arithmetic on thousands of random cases. A few results from those tests are worth knowing whatever tool you use.

Some Totals Cannot Happen

With sales tax rounded to the cent, not every total is possible. At 8.875%, a $45.91 price comes to $49.98 and $45.92 to $50.00, so a $49.99 receipt cannot come from that rate; in our test about 7.5% of random totals had no matching price. The Sales Tax Calculator says so instead of inventing a price. Discounts work the other way: at steep discounts several original prices round to the same sale price, which the Discount Calculator shows as a range.

Stacked Percentages Do Not Add

25% off and then an extra 10% off is not 35% off. The second discount comes off the reduced price: $80 becomes $60 and then $54, which is 32.5% off. The same rule applies to raises and price increases one after another, which multiply rather than add.

Splitting a Bill Exactly

$118.00 divided three ways is $39.333…; three shares of $39.33 leave a cent unpaid. The Tip Calculator gives one person $39.34 and two $39.33 so the shares add up to the bill.

Hourly Pay and Salary

$25 an hour for 40 hours a week over 52 weeks is $52,000 a year, $2,000 every two weeks and $4,333.33 a month. U.S. federal agencies divide a salary by 2,087 hours, not 2,080, as 5 U.S.C. 5504 requires, so $75,000 is $35.94 an hour there and $36.06 on the usual count. The Salary Calculator does both and shows which years have 27 biweekly paydays.

Loans: Where the Interest Goes

On a $300,000 mortgage at 6.5% over 30 years the payment is $1,896.20, and the interest over the whole loan is $382,636.71. The last payment is $1,900.91, a few dollars more, because the regular payment was rounded down to the cent. Extra principal early does the most: $200 a month from the start saves $103,450.19. See the Loan Calculator and Mortgage Calculator.

Growth, Inflation and Retirement

The Compound Interest Calculator shows the first, the Inflation Calculator the second from official Bureau of Labor Statistics data, and the Retirement Calculator the third, along with how long that money lasts at the income you want and the extra saving that would close a gap. Every growth figure depends on the rate you assume; real returns vary from year to year.

How We Checked

Each figure comes from the tools’ own code, which we tested against independent calculations in exact fractions or high-precision decimals: 2,007 loans and every row of their schedules, 4,006 sales-tax cases, 4,004 discount cases, 5,005 tip splits, 3,000 salary conversions, 1,507 compound-interest plans, 4,003 inflation conversions and 2,001 retirement plans. None is financial advice; they do the arithmetic on the numbers you give them.

Frequently Asked Questions

Published Oct 14, 2026 by Derek Giordano