30 hand-built tools · formulas & worked examples included

Calculators that explain the math, not just spit out a number.

Free, fast and accurate. Every Calcool tool runs instantly in your browser, shows the exact formula it uses, walks through a worked example, and cites where the method comes from.

📣 Marketing & Selling
Flagship
📣

CPM Calculator

Work out cost per 1,000 impressions, total ad spend, or reach for any budget.

💸

PayPal Fee Calculator

See the exact PayPal fee and what actually lands in your account.

🏷️

eBay Fee Calculator

Estimate eBay final-value fees and your real take-home per sale.

Percentage Calculator

Percent of a number, percentage change, and X is what % of Y.

💰 Money & Finance
📈

Investment Calculator

Project compound growth with regular contributions over any horizon.

🏦

Loan Calculator

Monthly payment, total interest and a full amortization schedule.

💼

NPV Calculator

Discount future cash flows to today and judge a project at a glance.

🧾

Sales Tax Calculator

Add or back out US sales tax at any rate, forward or reverse.

🏠

Mortgage Calculator

Monthly principal-and-interest payment and a full amortization schedule.

🚗

Car Payment Calculator

Monthly auto-loan payment from price, down payment, tax, APR and term.

💳

Credit Card Payoff Calculator

How many months to clear a balance, and the total interest you'll pay.

🏦

401(k) Calculator

Project your 401(k) at retirement, employer match included.

🌴

Retirement Calculator

Project savings vs the nest egg your income goal really needs.

💹

ROI Calculator

Return on investment, net profit and annualized ROI (CAGR).

📊 Business & Pricing
📊

Profit Margin Calculator

Gross profit, margin and the equivalent markup from revenue and cost.

📈

Markup Calculator

Set a selling price from cost and a markup, and see the resulting margin.

🏷️

Discount Calculator

Sale price and savings from a percent-off, including stacked discounts.

🧑‍💼 Work & Pay
💵

Paycheck Calculator

Estimate US take-home pay after federal tax, Social Security and Medicare.

⏱️

Time Card Calculator

Add daily clock-in/out times into weekly hours and gross pay.

🕒

Hours Calculator

Duration between two times, in hours-and-minutes and decimal hours.

🎓 School & Health
🎓

GPA Calculator

Compute a weighted GPA on the standard US 4.0 scale by credit hours.

📝

Grade Calculator

Weighted course grade and letter grade from assignment scores.

⚖️

BMI Calculator

Body mass index and weight category, in metric or US units.

🏃

Body Fat Calculator

Estimate body fat percentage with the US Navy tape method.

🍷

BAC Calculator

Estimate blood alcohol content with the Widmark formula (educational).

🧮 Everyday & Math
📉

Percentage Change Calculator

Percent increase or decrease between an old and a new value.

🍽️

Tip Calculator

Gratuity, grand total and the per-person share when splitting a bill.

🎂

Age Calculator

Your exact age in years, months and days, plus days lived.

📅

Date Difference Calculator

Days, weeks and months between two dates, business days optional.

📐

Square Footage Calculator

Area in square feet and the material cost per square foot.

Why Calcool is different

Most “free calculator” sites give you a box, a button and a number — and leave you guessing whether the answer is right or how it was reached. Calcool was built on the opposite idea: every tool is auditable. Under each calculator you'll find the precise formula it applies, a fully worked example with real numbers, an FAQ that answers the questions people actually search for, and links to the authoritative source the method comes from — the IRS, the SSA, the U.S. Department of Transportation, accounting standards bodies and similar.

The calculators run entirely in your browser. Nothing you type is uploaded, stored or shared — the math happens on your device the instant you click Calculate. That makes Calcool quick on any connection and private by default.

New tools are added regularly. If you'd like to see a calculator that isn't here yet, the contact details are on the about page.