Time Calculator
How to Add and Subtract Time (Hours and Minutes)
The time calculator adds durations, not clock times. Carry 60 seconds, keep hours past 24, and read decimal hours for a timesheet.
Calculator guides
Plain-English formulas, tables and examples for the searches people actually type — then a free tool to run your own numbers. Every guide matches a calculator on this site.
Showing 1–6 of 19 guides
Temperature Converter
How to Convert Temperature (F, C and Kelvin)
Temperature conversion needs an offset, not just a multiplier. Here are F↔C, kelvin, Rankine, a reference table and the free converter.
Slope Calculator
How to Find Slope From Two Points
m = rise/run from two points. Vertical lines are undefined. Here is y = mx + b, a table of cases and the free slope calculator.
Ratio Calculator
How to Simplify a Ratio (and Find Equivalents)
A ratio is part-to-part. Divide by the GCF to simplify, or scale both terms for an equivalent. The free ratio calculator does both.
Factor Calculator
How to Find Factors of a Number
Factors come in pairs up to the square root. Here is the list for 84, prime factorization, and the free factor calculator — not factorial n!.
Prime Number Checker
How to Check a Prime Number
Prime means exactly two positive divisors. Here is the square-root test, why 1 is not prime, and the free prime number checker.