Everyday Calculators
Percentages, dates, hours, averages — the quick ones.
The calculators you reach for most: percentages, age and date difference, hours worked, simple averages. This cluster bundles them with short explainers for the common gotchas (e.g. why percent change isn't symmetric).
Start here
- Percentage Calculator — Three modes: of, is-what-%, change.
- Age Calculator
- Date difference
Turn everyday calculators into number practice
Before revealing the exact answer, guess the rough result. Cognitive Boost uses that estimate-first habit inside Number Sense, Time and Focus, and Money Clarity circuits.
Featured tools
Percentage Calculator
Calculate percentages: what is X% of Y, X is what % of Y, and % change.
Percent Of Calculator
Calculate exactly what X% of Y equals, with quick-math anchors for 1%, 10%, 25%, and 50%.
Percent Increase / Decrease Calculator
Calculate the percent change from one value to another — increase or decrease — with exact delta and ratio.
Age Calculator
Calculate exact age in years, months, days, hours, and minutes.
Age On A Specific Date
Calculate how old you will be — or were — on any target date. Works for past and future dates.
Date Difference Calculator
Calculate the difference between two dates in years, months, weeks, days, hours, minutes, and seconds.
Hours Calculator
Calculate hours worked between two times, subtract an unpaid break, and optionally estimate pay.
Average Calculator
Calculate mean, median, sum, min, max, range, and standard deviation for any list of numbers.
Guides & explainers
- Everyday Calculators GuideUse quick calculators to check formulas, compare options, and practice estimating before the answer appears.
- How to Calculate Percentage IncreaseThe formula, the common mistakes, and when to use decrease instead.
- How to Calculate a Percent of a NumberMultiply by the decimal — that is the whole trick.
- Percentage Mental Math TricksFive shortcuts that cover tips, discounts, tax, and sale prices.
- How to Calculate Age on a DateBirthday, target date, answer — with the leap-year and timezone traps.
- How to Calculate Days Between Two DatesThe subtraction, the off-by-one, and how weekdays-only differ.
- Mental Math Practice With CalculatorsThe fastest way to turn a utility into a game is to guess before you reveal the answer.
- Percent Math Calculator GuideAll three percent questions in one place — "X is what % of Y?", "X% of Y = ?", and "% change from A to B?"
- Mean vs Median vs Mode GuideThree "averages" that can give wildly different answers on the same list — and how outliers decide which one is honest.
- Ratio and Proportion GuideHow ratios simplify, how proportions scale, and why "part-to-part" vs "part-to-whole" is the error that hides in half of all ratio problems.
- Fraction Arithmetic GuideThe four operations in a single page — plus the LCD trick that makes addition painless and the "keep-change-flip" rule for division.
- Mortgage Prepayment GuideWhy a small monthly extra payment beats a lump sum at year-end, and how biweekly-equivalent math cuts years off a 30-year loan.
Related clusters
Frequently asked questions
› Why isn't percent change symmetric? Troubleshooting
100 → 125 is +25%, but 125 → 100 is −20%. Percent change divides by the starting value, so the base changes when you reverse direction.
› Is 10% off, then 10% off, the same as 20% off? Trust & accuracy
No. Two 10% discounts compound to 19% off. Stacked discounts are always worth less than the simple sum.