This is the forward calculation: you have a starting figure and a rate, and you want to know where you land. Add 15% to a $80 price. Give a $62,000 salary a 4% rise. Grow a 1,200-unit forecast by 8%. The answer is always the original plus the extra, and there is a shortcut that makes it a single multiplication.
Rather than working out the increase and then adding it, multiply by 1 plus the rate as a decimal. A 15% increase is × 1.15. A 4% increase is × 1.04. A 150% increase is × 2.5. This “growth factor” approach is not just faster — it is what makes multiple changes easy to chain together, because factors multiply cleanly while percentages do not add.
When you need this
Applying a price rise. New list prices at the start of a year, an inflation adjustment on a contract, a supplier passing through a cost increase.
Adding a charge on top. Sales tax, VAT, GST, a service fee, a card surcharge. These are all “increase by a percentage” operations — though tax specifically has its own sales tax calculator with the rate handling built in.
Projecting growth. If a figure grows 8% a year, next year is × 1.08 and five years out is × 1.08⁵. Compounding is just repeated application of the same factor.
Worked examples
$80 increased by 15%. Multiply 80 by 1.15 to get $92. The long way — 15% of 80 is 12, and 80 + 12 = 92 — gives the same answer and is a good check.
$62,000 increased by 4%. 62,000 × 1.04 = $64,480. The rise itself is $2,480.
1,200 increased by 8%. 1,200 × 1.08 = 1,296.
Two increases in a row. A price rises 10% and then a further 5%. That is × 1.10 × 1.05 = × 1.155, so a total rise of 15.5%, not 15%. The second rise applies to the already-raised price, so it earns a little more than its headline rate. Over many periods this gap becomes substantial — it is the whole mechanism behind compound interest.
Where people go wrong
Adding the rate instead of the factor. Multiplying 80 by 1.15 is right; multiplying by 0.15 gives you only the increase, and multiplying by 15 gives nonsense. If you use the decimal-only version, remember you still have to add it back to the original.
Reversing an increase by subtracting the same percentage. Add 15% to $80 and you get $92. Take 15% off $92 and you get $78.20, not $80. To undo an increase you divide by the factor, not subtract the rate — which is exactly what the reverse percentage calculator does.
Summing rates across periods. Three years of 8% growth is not 24%. It is 1.08³ = 1.2597, so 26%. Small per-period differences compound into meaningful gaps over time.
Confusing this with “what percentage increase was that?” If you already know the before and after and want the rate, you need the percentage increase calculator instead. This page goes forwards; that one goes backwards. Our guide on percentage increase versus increase by a percentage is the one to read if the two keep blurring together.
Applying a raise to the wrong base. A 3% cost-of-living rise followed by a 5% promotion increase gives 1.03 × 1.05 = 8.15%, not 8%. Whether the second is applied to the raised salary or the original is a real question with a real dollar difference, and it is worth being explicit about in writing.
Doing it in your head
The factor method works mentally too. For a 10% rise, take a tenth and add it. For 5%, take a tenth and halve it. For 15%, do both. For 20%, take a fifth. Anything built from 10%, 5% and 1% can be assembled quickly — the percentages in your head guide has the full set.