🧮 Guide hub · 31 tools · 36 guides · Updated October 7, 2026
Calculator guides: percentages, money, units and time zones
Each guide shows the formula first, with a worked example you can check by hand, then how to get the same answer instantly with the matching calculator. Knowing the formula is what lets you spot a wrong result, such as a stacked discount or a reverse tax calculation done the wrong way.
Calculators run in your browser. Figures such as tax rates and exchange rates change over time; the tools state their source or update frequency, and financial results are estimates, not financial or tax advice.
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Calculate a Percentage: The Formula With Worked Examples
Calculate a percentage with one formula: part divided by whole, times 100. Worked examples for grades, discounts…
How to Calculate a Tip: One Formula for Any Rate
Calculate a tip on any bill with one formula: bill times rate. Mental shortcuts for 18% and 22%, pre-tax vs total, and…
Mortgage Payment Calculator: Compare Rates and Quotes
A free mortgage payment calculator for comparing lender quotes: what each rate step costs, points break-even, Loan…
Compound Interest Calculator for Savings Goals
Use a compound interest calculator for savings: an emergency fund, a down payment goal, deposit timing, yearly raises…
Convert Units Correctly: Gallons, Ounces, Tons and More
Convert units without the classic traps: US vs imperial gallons, fluid vs weight ounces, three kinds of ton, squared…
Convert PST to EST: The 3-Hour Rule and DST Traps
Convert PST to EST fast: Eastern is 3 hours ahead of Pacific. See PST vs PDT, the rare overnight DST hours, Arizona's…
Which tool do I need?
| If you want to… | Use |
|---|---|
| Work out X% of a number or a percent change | Percentage Calculator |
| Check a sale price or stacked coupons | Discount Calculator |
| Add or remove sales tax or VAT | Sales Tax & VAT Calculator |
| Estimate a monthly mortgage payment | Mortgage & Loan Calculator |
| Estimate a car payment with trade-in and sales tax | Auto Loan Calculator |
| See how savings grow | Compound Interest Calculator |
| Convert cm, lb, °F, gallons and more | Unit Converter |
| Find a meeting time across time zones | Time Zone Converter & Meeting Planner |
| Count days between two dates | Age Calculator |
| Solve an expression with sin, log, powers or roots | Scientific Calculator |
| Find the standard deviation, mean and quartiles of a list | Standard Deviation Calculator |
| Estimate a personal loan payment, APR with fees or payoff time | Loan Calculator |
| Project 401(k) savings and retirement income | Retirement Calculator (401k) |
| Estimate body fat % from tape measurements | Body Fat Calculator |
| Find how long a credit card takes to pay off | Credit Card Payoff Calculator |
| Measure square footage of a room or lawn | Square Footage Calculator |
| Work out cubic yards and bags of concrete | Concrete Calculator |
| Estimate mulch or gravel for a garden or driveway | Mulch Calculator |
Free tools in this hub
Percentage Calculator
Calculate percentages quickly and easily.
Discount Calculator
Sale price after % or $ off, stacked discounts, what % off it was, BOGO and “3 for $10” deals, with optional sales tax.
Tip Calculator
Calculate the tip and split the bill between people.
Sales Tax & VAT Calculator
Add, remove or find sales tax and VAT — US state and EU presets, itemized receipts.
Mortgage & Loan Calculator
Full monthly payment with property tax, insurance, PMI and HOA (PITI), extra-payment savings, payoff date and amortization chart.
Auto Loan Calculator
Monthly car payment with trade-in, down payment, state sales tax and fees — total interest, term comparison, amortization schedule and CSV.
Loan Calculator
Monthly payment, how much you can borrow or payoff time for a personal loan — origination fee and effective APR, extra payments…
Credit Card Payoff Calculator
See how long it takes to pay off a credit card and the interest you pay — minimum payment vs a fixed payment or a payoff date, with a…
Retirement Calculator (401k)
Project your 401(k) or retirement savings with raises, employer match and the 2026 IRS limit — balance at retirement, today’s dollars, 4%…
Compound Interest Calculator
See how your savings grow with monthly contributions.
Paycheck & Salary Calculator
Take-home pay after 2026 federal tax, Social Security and Medicare — salary or hourly with overtime, 401(k) and state estimate, salary ↔…
Currency Converter
Convert between currencies with live exchange rates.
Unit Converter
Convert 15 kinds of units with exact factors — US and imperial volume, cooking, fuel economy (mpg ↔ L/100 km), data (MB vs MiB), energy…
Square Footage Calculator
Find square feet for rooms, lawns and floors — rectangles, circles, triangles and L-shapes, feet, inches or meters, with waste and cost per…
Concrete Calculator
How much concrete for a slab, footing, post hole, column or stairs — cubic yards, 40/60/80 lb bags, weight and cost, with extra for waste.
Mulch Calculator
How much mulch you need for garden beds and tree rings — cubic yards and 2 or 3 cu ft bags at any depth, with bag vs bulk cost.
Gravel Calculator
Tons and cubic yards of gravel, crushed stone, pea gravel, river rock or sand for a driveway, path or patio — by area, depth and material.
Time Zone Converter & Meeting Planner
Find a meeting time across time zones — type any city or PST/IST, see a 24-hour working-hours grid, the best overlap, DST handled…
Age Calculator
Exact age, days between dates, business days, add/subtract dates and countdowns.
BMI Calculator
Calculate your Body Mass Index instantly.
GPA Calculator
Semester and cumulative GPA on the 4.0 scale with +/− grades, weighted Honors/AP, and the grades you need to hit a target GPA.
Grade Calculator
Weighted course grade from percents or points, what you need on the rest of the class, and the final exam score you need for an A, B or C.
Scientific Calculator
Free online scientific calculator with sin, cos, tan, logs, powers, roots, factorials and percent — DEG/RAD, memory, history and full…
Standard Deviation Calculator
Paste a list of numbers for sample and population standard deviation, variance, mean, median, mode, quartiles, outliers and z-scores, with…
Fraction Calculator
Add, subtract, multiply and divide fractions and mixed numbers with steps; simplify, and convert decimals (even repeating) to fractions.
Time Card Calculator
Add up work hours from clock-in and clock-out times with unpaid breaks, overnight shifts, rounding, overtime and gross pay. CSV export.
Calorie Calculator
Daily calories to maintain, lose or gain weight (TDEE) with Mifflin-St Jeor, Harris-Benedict or Katch-McArdle, US or metric units, plus…
Body Fat Calculator
Estimate body fat % with the U.S. Navy tape method (US or metric), ACE category, fat and lean mass, and a BMI-based estimate for comparison.
Roman Numeral Converter
Convert numbers to Roman numerals and back with a place-value breakdown, standard-form check, overline numbers to 3,999,999 and dates.
Random Number Generator
Secure random numbers in any range with no repeats, pick winners from a list, split into teams, roll dice and flip coins. Private, in your…
Binary, Hex & Decimal Converter
Convert binary, decimal, hex, octal and any base 2–36 with no size limit, two’s complement, worked steps, and text to binary or hex.
Unix Timestamp Converter
Convert Unix timestamps (seconds, ms, µs, ns — auto-detected) to dates in any time zone and back: ISO 8601, RFC 2822, relative time, ISO…
Bank Statement to Excel
Convert bank statement PDFs to Excel or CSV — dates, descriptions, debit/credit and a running-balance check that flags rows that don't add…
Guides by tool
Percentage Calculator
Open tool →Discount Calculator
Open tool →Tip Calculator
Open tool →Sales Tax & VAT Calculator
Open tool →Mortgage & Loan Calculator
Open tool →Compound Interest Calculator
Open tool →Currency Converter
Open tool →Unit Converter
Open tool →- Celsius to Fahrenheit: Formula, Oven Chart and Fevers
- Celsius to Kelvin: Add 273.15, and Why Science Cares
- Convert Units Correctly: Gallons, Ounces, Tons and More
- Fahrenheit to Celsius: Formula, Chart and Travel Anchors
- How to Convert cm to Inches (Formula + Quick Trick)
- How to Convert Feet to Meters (Formula + Quick Trick)
- How to Convert Inches to cm (Formula + Quick Trick)
- How to Convert Kilograms to Pounds (Formula + Quick Trick)
- + 6 more in all guides
Time Zone Converter & Meeting Planner
Open tool →Age Calculator
Open tool →BMI Calculator
Open tool →Frequently asked questions
Are the calculators accurate?
They use exact formulas and exact conversion factors, for example 1 inch = 2.54 cm and 1 lb = 0.45359237 kg. Results are rounded for display; money results are estimates that depend on the inputs you give.
Do the finance calculators give financial advice?
No. They show the math behind a payment, a savings plan or a paycheck so you can compare options. Talk to a qualified adviser before a major decision.
How current are exchange rates and tax rates?
Currency rates are reference rates refreshed about once a day, not the rate your bank or card will charge. Tax presets reflect published rates at the time the tool was last updated; always check your local tax authority.
Why do my percent-change answers differ from a friend's?
Percent change depends on which value is the base. Going from 50 to 75 is a 50% increase, while going from 75 back to 50 is a 33.3% decrease.
Do time zone conversions handle daylight saving?
Yes. The converter uses your browser's time zone database, so it applies the correct offset for the date you pick, including the weeks when the US and Europe change clocks on different Sundays.
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