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Plain-English explainers for the formulas behind our calculators, each with a worked example and links to the tools.
Showing 12 guides
Math · 1 min read
How to calculate percentages
Most percentage questions come down to three formulas. Learn which one fits your question, then check the answer with the calculator.
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Education · 1 min read
How to calculate your GPA
A GPA is a weighted average of your grade points. Here is how the arithmetic works, and where schools differ.
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Date & Time · 1 min read
How to calculate age from a date of birth
Age in whole years is a subtraction with one check. Exact age in years, months and days needs a little more care.
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Finance · 1 min read
How to convert an hourly wage to an annual salary
Multiply your hourly rate by the hours you work in a year. The only choices are how many hours and how many weeks.
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Auto & Travel · 1 min read
How to calculate fuel economy and trip cost
Fuel economy is distance divided by fuel. Once you know it, trip cost is one more step.
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Home & DIY · 1 min read
How much paint do I need for a room?
Measure the walls, subtract the openings, multiply by coats and divide by the coverage on the can.
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Math · 1 min read
How to calculate percentage change
Percentage change compares a new value with an old one. Subtract, divide by the old value, then multiply by 100.
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Education · 1 min read
How to calculate your final grade
Your course grade is a weighted average. Knowing the weight of the final exam lets you solve for the score you need.
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Date & Time · 1 min read
How to calculate the days between two dates
Subtract the earlier date from the later one. Leap years and whether you count both end dates are the only details to watch.
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Finance · 1 min read
How to calculate loan payments
A fixed-rate loan has a level monthly payment. The formula needs only the amount, the rate and the number of months.
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Conversions · 1 min read
How to convert grams to cups
A cup measures volume and a gram measures weight, so the conversion depends on the ingredient. Divide the grams by that ingredient's grams per cup.
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Finance · 2 min read
How to calculate compound interest
Compound interest earns interest on your earlier interest. The formula needs the starting amount, the rate, how often it compounds and the time.
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