Percentage Calculator

How does the percentage calculator work?

A percentage is a fraction of 100. Work out 15% of 200 and you multiply 200 by 15, then divide by 100, which gives 30. Every mode here is a spin on that one idea, and the tool always prints the exact line of arithmetic it used, so you can check it or drop it into homework and reports.

The five modes at a glance

How do I add or subtract a percentage?

Increases and decreases are the everyday cases: a 25% raise on an 80 dollar price, a 30% cut on a 149 dollar jacket. The calculator first works out the percentage amount, then adds or subtracts it. For the jacket, 30% of 149 is 44.70, so you pay 104.30. Mind the classic trap: an increase and a decrease of the same percentage do not cancel out. Start at 100, add 25% to reach 125, strip 25% back off and you land on 93.75, not 100, because the second 25% is measured on a bigger base.

How do I find the percent change between two values?

The percent change mode answers questions like "my rent went from 1,200 to 1,320, what is the increase?". The formula divides the difference by the starting value: (1320 - 1200) ÷ 1200 × 100 = 10%. A negative result flags a drop. Reach for this mode on price history, salary moves, traffic reports or any before-and-after comparison.

What percent is A of B?

The last mode runs the question backwards. You scored 42 points out of 60: 42 ÷ 60 × 100 = 70%. It also covers market share (3,400 sales out of 27,000 total is 12.59%) and progress tracking (8,540 dollars raised toward a 10,000 goal is 85.4%).

How precise are the results?

Every calculation runs at full floating-point precision, and only the value on screen is rounded, to at most 6 decimal places. So one third of 100 shows as 33.333333, yet chained calculations never pick up rounding drift. Inputs from -1,000,000,000,000 to 1,000,000,000,000 are accepted, negatives included, so you can measure changes on losses or temperatures below zero.