How this calculator works
Multiply the starting value by the decrease percentage as a fraction, then subtract that amount. To solve for the rate, divide the amount removed by the starting value. The decrease rate must be non-negative; percentages above 100 are allowed.
Formula
Decrease = old × r/100. New = old - decrease. Decrease rate (%) = (old - new) / old × 100.
Worked example
Removing 25% of 200 removes 50 and leaves 150. Removing 100% removes all 200 and leaves zero. Removing 150% leaves -100.
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Frequently asked questions
Can a decrease be more than 100%?
Yes, mathematically. For a positive starting value, a decrease above 100% produces a negative result. Whether that is useful depends on the quantity being modelled.
Can I undo a 25% decrease with a 25% increase?
No. After 200 falls to 150, a 25% increase gives 187.5. Returning to 200 requires an increase of one third.
What if my new value is higher than the starting value?
The solve-for-rate mode reports a negative decrease, which is an increase, and rejects it. Use the percentage increase page for that case.