Ratio & Proportion Solver
How it works
A:B = C:D means A/B and C/D are the same ratio. Cross-multiplying clears the fractions to give A×D = B×C, one equation relating all four terms. Whichever term is missing, that same equation rearranges to isolate it — dividing the product of the two terms diagonally opposite it by the one term remaining on its own side.
Frequently asked questions
What is a proportion, in plain terms?
A proportion is two ratios set equal to each other — a statement that "A is to B" the same way "C is to D". "2 is to 4 as 3 is to what?" is a proportion with one term missing, and this tool solves for whichever of the four terms you leave blank.
Why does cross-multiplication solve it?
A:B = C:D is the same statement as the fraction equation A/B = C/D. Cross-multiplying — multiplying each side by the other denominator — turns that into A×D = B×C, a single equation with no fractions in it, which can be rearranged to isolate any one of the four terms once the other three are known.
Can you walk through an example?
Take 2:4 = 3:x. Cross-multiplying gives 2 × x = 4 × 3, which is 2x = 12, so x = 6 — matching the intuition that 2:4 and 3:6 are both the ratio "one half".
Other tools
Utilities
- Aspect Ratio CalculatorGet width or height for a ratio like 16:9.
- Password GeneratorStrong random passwords in your browser.
- Number to WordsWrite a number out in words, for cheques and forms.
- Roman Numeral ConverterNumbers to Roman numerals and back, either direction.
- Temperature ConverterCelsius, Fahrenheit and Kelvin, all at once.
- Pace & Speed ConverterPace and speed for running, plus race finish times.
- CGPA to Percentage CalculatorAn approximate percentage from a CGPA, on any scale.
- Length ConverterMillimetres to miles, and everything between, all at once.
- Weight ConverterMilligrams to stone, and everything between, all at once.
- Number Base ConverterConvert an integer between any two bases from 2 to 36.
- Data Storage ConverterBytes to KB, MB, GB, TB and back, binary or decimal.
- Volume ConverterMillilitres to gallons, and everything between, all at once.
- Statistics CalculatorMean, median, mode and standard deviation of a list.
- Random Number GeneratorRandom numbers in a range, unique or repeating.
- Dice RollerRoll any number and size of dice, with the total.
- Screen Resolution / PPI CalculatorPixel density from a screen size and resolution.
- Data Bandwidth CalculatorDownload time from a file size and connection speed.
- Paper Size ConverterDimensions for A4, Letter, Legal and other paper sizes.
- Speed, Distance & Time CalculatorSolve for speed, distance or time from the other two.
- Flooring & Tile CalculatorBoxes of flooring or tile needed for a room, with waste.
- Percentage Change CalculatorPercent increase or decrease between two values.
- Fraction CalculatorAdd, subtract, multiply or divide fractions, simplified.
- Golden Ratio CalculatorThe golden-ratio proportioned length for any dimension.
- Betting Odds ConverterConvert odds between fractional, decimal, American and probability.
- Pet Food Portion CalculatorDaily food amount for a dog or cat by weight and activity.
- Pressure ConverterPSI, bar, pascal and atmospheres, all at once.
- Energy Unit ConverterJoules, calories, kWh and BTU, all at once.
- Power Unit ConverterWatts, horsepower and BTU/hr, all at once.
- Angle ConverterDegrees, radians and gradians, all at once.
- Concrete CalculatorConcrete volume and bags needed for a slab.
Finance
- EMI CalculatorLoan EMI, amortization schedule and prepayments.
- SIP CalculatorWhat a monthly investment grows to over time.
- Compound Interest CalculatorMaturity value of a one-time deposit, any compounding.
- GST / Sales Tax CalculatorAdd tax to a price or work back from a total.
- Loan Affordability CalculatorThe most you can borrow at a monthly payment you can afford.
- Simple Interest CalculatorInterest and total for a fixed rate over time.
- CAGR CalculatorCompound annual growth rate from a starting and ending value.
- Investment Doubling Time CalculatorYears to double your money at a rate, or the rate needed to double it by a target year.
- Mortgage CalculatorMonthly payment including property tax, insurance and PMI.
- Retirement Savings CalculatorWhat regular contributions grow to by retirement.
- Debt Payoff CalculatorSnowball or avalanche order to clear multiple debts.
- Savings Goal CalculatorMonthly deposit needed to reach a savings target.
- Inflation CalculatorWhat an amount from one year is worth in another.
- Break-Even Point CalculatorUnits or revenue needed to cover fixed and variable costs.
- Profit Margin CalculatorGross margin and markup from cost and selling price.
- Exchange Rate CalculatorConvert an amount using an exchange rate you supply.
- Net Worth CalculatorTotal assets minus liabilities, with a full breakdown.
- Emergency Fund CalculatorHow many months of expenses your savings would cover.
- Credit Card Minimum Payment CalculatorHow long minimum payments take, versus a fixed payment.
- Rent Affordability CalculatorRent you can afford from your income, by the 30% rule.
- Freelance Hourly Rate CalculatorHourly rate to charge from a target income and billable hours.
- Property Tax CalculatorAnnual and monthly property tax from a home value and rate.
- Auto Loan CalculatorMonthly car payment with trade-in, down payment and sales tax.
- Zakat CalculatorZakat due at 2.5% of qualifying assets above the nisab.
- Loan-to-Value (LTV) CalculatorLoan-to-value ratio from a loan amount and property value.
Everyday
- Percentage CalculatorFind X% of a number and more.
- Tip CalculatorTip amount, total and per-person split.
- Fuel Cost CalculatorWhat a trip costs in fuel, split any number of ways.
- Date Difference CalculatorDays and weeks between two dates.
- Age CalculatorYour exact age in years, months and days, plus your next birthday.
- Discount CalculatorSale price from a discount, or the original price from a sale.
- Unit Price CalculatorCompare two pack sizes and see which is cheaper per unit.
- Time Duration CalculatorAdd or subtract two durations, normalized and signed.
- Time Zone ConverterConvert a time between two time zones.
- Countdown CalculatorLive countdown to any date and time.
- Group Expense SplitterSplit shared costs unevenly among named people.
- Work Hours CalculatorTotal hours worked from clock-in and clock-out times.
- Grade CalculatorWeighted final grade from assignment scores.
- Recipe Ratio ScalerScale ingredient quantities up or down for any serving size.
- Gas Mileage CalculatorFuel economy in MPG or L/100km from odometer and fuel used.
- Paint Coverage CalculatorHow much paint a room needs, from its dimensions.
- Commute Cost CalculatorMonthly commute cost by car versus public transit.
- Pet Age CalculatorA dog or cat's age in human years.
- Party Food & Drink PlannerHow much food and drink to get for a party, by guest count.
- Appliance Energy Cost CalculatorWhat running an appliance costs, from its wattage and use.
- Rent Split CalculatorSplit rent fairly among roommates by room size or evenly.
- Reading Pace CalculatorPages a day needed to finish a book by a target date.
- Shopping Cart Total CalculatorAdd up a list of items with quantities and tax.
- Days Since / Milestone CalculatorDays, weeks or months since a date, and upcoming milestones.
- Random Team GeneratorShuffle a list of names into random teams or a winner.
Health
- BMI CalculatorBody mass index and category, in metric or imperial.
- Calorie CalculatorDaily calories (BMR and TDEE) from your stats and activity.
- Pregnancy Due Date CalculatorEstimated due date and current week, from your last period.
- Body Fat Percentage CalculatorEstimate body fat percentage from tape measurements, U.S. Navy method.
- Water Intake CalculatorDaily water target in litres and glasses from body weight and exercise.
- Ideal Weight CalculatorReference body weight from four classic formulas, side by side.
- Macro CalculatorProtein, carb and fat grams from a calorie target.
- Heart Rate Zone CalculatorTraining heart rate zones from your age.
- Waist-to-Hip Ratio CalculatorWaist-to-hip ratio and health risk category.
- Sleep Cycle CalculatorBedtimes or wake times aligned to 90-minute cycles.
- One-Rep Max CalculatorEstimated one-rep max from any weight and reps.
- Body Surface Area CalculatorEstimated body surface area from height and weight.
- Blood Alcohol Content EstimatorEstimated BAC from drinks, weight, sex and time.
- Standard Drink CalculatorHow many standard drinks a beverage contains.
- Intermittent Fasting CalculatorYour eating window and next fast, from a fasting schedule.
- Ovulation CalculatorEstimated fertile window from your cycle length.
- Steps to Calories CalculatorCalories burned walking, from your step count and weight.
- Blood Pressure Category CheckerBlood pressure category from a systolic/diastolic reading.
- Caffeine Half-Life CalculatorHow much caffeine is still in your system over time.