Calculator Guides
In-depth articles on how to calculate everything — from compound interest to BMI, mortgage payments and beyond.
What Is the Rule of 72? Double Your Money Math
Learn the Rule of 72: divide 72 by your interest rate to estimate how fast money doubles. Worked examples, reverse math, and when this shortcut breaks down.
How to Calculate Your Due Date
Learn how to calculate your pregnancy due date using Naegele's rule, conception dates, and ultrasound dating, plus why babies rarely arrive exactly on time.
Maximizing Your 401(k) Employer Match
Learn how 401(k) employer match formulas work, exactly how much to contribute to capture every dollar, and how vesting schedules affect your free money.
Quadratic Formula: How to Solve Quadratic Equations
The quadratic formula step by step, what the discriminant tells you about the roots, when factoring is quicker, common mistakes, and the site's calculators.
What Is a Healthy Body Fat Percentage?
What is a healthy body fat percentage? See healthy ranges by age and sex, why men and women differ, and how to interpret your own number safely.
How to Convert Your Hourly Wage to an Annual Salary
How to convert an hourly wage to an annual salary with a formula and worked examples, plus taxes, unpaid time off and comparing job offers fairly.
One Rep Max Calculator: How to Calculate Your 1RM
How the Epley, Brzycki, McGlothin and Lombardi formulas estimate a one-rep max, how accurate they are, safe 1RM testing, and training with percentages.
Heart Rate Zones: How to Calculate and Train in Each Zone
How to calculate five heart rate training zones from maximum heart rate or the Karvonen method, what each zone is for, and how to plan a training week.
How to Calculate a Tip (and Split the Bill)
A fast mental-math method for 15, 18 and 20% tips, pre-tax vs post-tax, splitting a bill evenly or by what each person ordered, and customary rates.
Ideal Weight Calculator: What Should You Weigh?
How the Hamwi and Devine ideal-weight formulas work, how they compare with the BMI healthy range, and why frame size and muscle change the answer.
Macro Calculator: How to Calculate Your Macros
How to turn a daily calorie target into protein, carbohydrate and fat grams, with worked examples of common splits and how the macro calculator sets each one.
What Is CAGR? Compound Annual Growth Rate Explained
Learn what CAGR (compound annual growth rate) means, how to calculate it with a worked example, and how to use a CAGR calculator to compare investments.
BMR vs TDEE Explained: Which Number Should You Use?
The difference between BMR and TDEE, how activity multipliers connect them, a worked example, and which number to use for weight loss, maintenance or gain.
How to Calculate Your GPA: Weighted and Unweighted
How to calculate a GPA step by step: convert letter grades to points, weight by credit hours with quality points, and tell weighted from unweighted GPA.
BMI Calculator: What It Is and How to Use It
How BMI is calculated from height and weight, the CDC/WHO adult categories, what BMI misses, and how to use it to track change over time.
Inflation and Your Savings: How Prices Erode Your Money
How inflation is measured, what $10,000 in cash is worth after 10 and 20 years, nominal vs. real returns, and how to keep savings ahead of inflation.
How to Calculate Interest Rate on a Loan
How loan interest and APR work, the payment formula with a worked auto-loan example, how to back out the rate from a payment, and how to compare offers.
Paycheck Calculator: How to Calculate Your Take-Home Pay
How federal and state income tax, FICA and benefit deductions turn gross pay into take-home pay, with 2026 figures and worked examples.
How to Build an Emergency Fund: A Step-by-Step Guide
How much to keep in an emergency fund, where to keep it, and how to build it from zero with automatic transfers, with a worked example.
VAT Calculator: How to Add and Remove VAT
How to add VAT to a net price and remove it from a gross price, why subtracting the rate gives the wrong answer, and worked invoice examples.
FIRE Explained: Financial Independence, Retire Early
How FIRE works: the 4% rule and the 25x FIRE number, savings-rate math with a worked example, the Lean, Fat, Coast and Barista variants, and the main risks.
How to Calculate Percentages: Every Method Explained
The percentage formula, converting between percents, decimals and fractions, percent of a number, percentage change vs. difference, and percentage points.
Calorie Deficit: How to Calculate and Use It for Weight Loss
How to estimate your daily calorie needs, size a moderate calorie deficit, and adjust it as you lose weight, with worked examples.
How to Calculate Your Monthly Mortgage Payment
The mortgage payment formula, a worked example, and how loan amount, rate, term and down payment change your monthly payment.
How to Calculate Compound Interest: Formula and Examples
The compound interest formula A = P(1 + r/n)^(nt), two worked examples, and how time, rate and compounding frequency change the result.
Torque: Rotational Force from Force and Distance
Calculate torque as force × lever arm (N·m or lb·ft), handle angled forces and unit conversions, and apply it to bolts, levers and motor ratings.
2x2 Matrix Determinant: Calculate It and Read It
Calculate a 2×2 determinant (a11·a22 − a12·a21), what zero or negative results mean for invertibility and orientation, and how it feeds Cramer's rule.
DPS: Calculate Damage Per Second and Compare Builds
Calculate damage per second from hit damage and attack speed, convert attack intervals to a rate, fold in crits and multipliers, and compare weapons fairly.
Aircraft Fuel Consumption: Fuel Cost of a Flight
Estimate a flight's fuel cost from distance in nautical miles, a per-mile burn rate by aircraft class, headwind, altitude and fuel price, with an example.
Beam Deflection: How Much a Beam Bends Under Load
Calculate the maximum deflection of a simply supported beam for a point load or a uniform load, with worked steel-beam examples and L/360 checks.
Power Consumption: What a Device Costs to Run
Turn a device's wattage, daily hours and your electricity rate into kilowatt-hours and a monthly cost, with a space-heater example and standby power.
Electrical Power: Watts from Voltage and Current
Calculate electrical power with P = V × I, find the current a load draws, size breakers and supplies (including the 80% rule), and when power factor matters.
Shipping Cost: Estimate Freight From Weight and Distance
Estimate a parcel's shipping cost from billable weight (actual or dimensional), distance, service level and a base fee, plus the surcharges a model omits.
Currency Hedging Cost: The Price of FX Protection
Estimate the all-in cost of hedging FX exposure with a forward: the forward-spot differential, the broker spread and a holding cost, with a worked example.
Crypto Portfolio Value: What Your Holdings Are Worth
Calculate crypto portfolio value (coins × current price, summed across holdings), read the allocation it reveals, and keep it accurate across wallets.
Recipe Scaling: Ingredient Amounts for Any Servings
Scale a recipe up or down by desired ÷ original servings, add a waste buffer, and handle what doesn't scale linearly: salt, leavening, time, pans.
Stellar Luminosity from Radius and Temperature
Compute a star's luminosity in solar units from radius and surface temperature with the Stefan-Boltzmann law, with a worked example and constants.
Vector Dot Product: How to Calculate It
Compute the dot product of two 3D vectors, read the sign and size of the result, and turn it into the angle between vectors or a projection.
Flesch Reading Ease: Calculate the Readability Score
A practical guide to the Flesch Reading Ease score — how to calculate it from words, sentences, and syllables, with a worked example and tips to write more clearly.
Years Between Events: Time Span Between Two Dates
How to find the years between two historical dates, including the missing year zero at the BC/AD boundary, with worked examples (1903 to 1969; 753 BC to AD 476) and timeline tips.
Map Scale: Convert Map Distance to Real Distance
Convert a distance measured on a map into real kilometres, metres or miles from the scale ratio, with a 1:50,000 worked example, the USGS 1:24,000 standard, and the unit mistakes to avoid.
K/D Ratio: Calculate and Use Your Kill-to-Death Ratio
How to calculate your kill-to-death ratio, how the zero-death case is handled, how K/D differs from KDA and rating systems, and how to use it to improve rather than just compare.
Flight Delay Compensation: What EU261 and UK261 Pay
How fixed flight delay compensation is calculated under EU261 and UK261: the 3-hour arrival test, the €250/€400/€600 distance bands, the 50% reduction for long-haul delays under 4 hours, and the extraordinary-circumstances rule.
Irrigation Water Requirement: Daily Water Needs for a Crop
Calculate a field's daily irrigation requirement from crop coefficient × reference evapotranspiration, effective rainfall, system efficiency and acres, in gallons per day, with a worked corn example.
Fantasy Draft Value: Is a Pick a Reach or a Steal?
How the draft value score weighs a player's overall rank, positional scarcity, league size and the gap to your pick, with a worked example, and how it relates to value-based drafting.
Household Water Usage: Estimate and Cut Your Consumption
Estimate weekly household water use from showers, toilets, laundry and dishwashing with a worked example, compare it with EPA averages, and see which fixtures and habits save the most.
Cooking Time Calculator: Roast Times by Weight
How a minutes-per-pound cooking time estimate is built from weight, method, doneness and oven temperature, with a worked roast example and the USDA safe temperatures to confirm with a thermometer.
Soil Bearing Capacity: Terzaghi's Equation Explained
How to estimate the ultimate bearing capacity under a square footing with Terzaghi's equation, which bearing-capacity factors the calculator uses, a worked example and the safety factor.
Swiss Tournament Points: The Score You Still Need
How to turn a target score percentage in a Swiss chess tournament into the points you still need, with a 9-round worked example and the strategy that follows from the gap.
Rectangular Rule: Approximating an Integral With Rectangles
How the rectangular (Riemann sum) rule approximates a definite integral, with a worked midpoint example, error behavior for left, right and midpoint heights, and common mistakes.
Expense Percentage: What Share of Income Each Cost Takes
Divide an expense by income to see what share of your pay it takes, then compare it with the 50/30/20 rule and HUD's 30% housing threshold. Worked example included.
Orbital Velocity: How to Calculate Orbit Speed
Calculate the speed of a satellite, moon or planet from the central mass and orbital radius with v = √(GM/r), with a worked ISS example and the eccentricity term explained.
Bed Adhesion Temperature: Picking a Heated Bed Setting
How to choose a starting heated-bed temperature for PLA, ABS, PETG and TPU, how room temperature shifts it, and how to fine-tune from a test layer.