THE NUMORIX GUIDE
How to use the Percentage Calculator
Last reviewed September 12, 2026
What this percentage calculator does
The calculator supports common percentage questions: finding a percentage of a number, finding what percentage one number is of another, measuring an increase or decrease, and estimating a tip. Each mode names the base value so the same two numbers are not accidentally interpreted in different ways.
Percentage formulas
Percentage of a number: result = percentage / 100 x base. What percentage: percentage = part / base x 100. Percentage change: change = (new value - original value) / original value x 100. A tip uses bill x tip rate, then adds the tip to the bill when a total is needed. The inputs are percentage, base, part, original value, new value, bill amount, and tip rate depending on the selected mode.
Worked examples
15% of $240 is 0.15 x 240 = $36. If $36 is compared with a $240 base, it represents 15%. A change from $240 to $276 is a 15% increase; a change from $240 to $204 is a 15% decrease. In each case, the original value is the comparison base.
How to interpret the result
A percentage is a relative comparison, not an amount by itself. A 20% change on $50 and a 20% change on $5,000 have the same rate but very different dollar effects. Percentage increases and decreases are not always reversible because their bases differ. For discounts and tax, confirm whether the displayed percentage applies before or after the other adjustment.
Common mistakes to avoid
Identify the original or base number before calculating. Do not use the new value as the denominator when the question asks for change from the original. Keep a percentage such as 15% separate from its decimal form, 0.15, until the multiplication step.
Assumptions and limitations
The calculator performs arithmetic from the mode and values entered. It does not decide whether a price, tax rate, tip, discount, or growth assumption is appropriate for a particular transaction or jurisdiction.