Proportion Calculator
Solve proportions online for free with our proportion calculator. Enter any three values in A / B = C / D and instantly find the fourth using cross-multiplication. Handles recipe scaling, unit conversion, percentage problems, and more — fast, accurate, and no signup required.
Solve proportions (A/B = C/D) by entering three known values, or use the Percentage of a Whole tab to find a part, percentage, or whole. All calculations run locally in your browser — no signup required.
Fill in 3 of 4 values — leave one blank to solve for it. (0/3 filled)
Why Use Our Proportion Calculator?
Instant Proportion Calculations
Solve proportions in milliseconds directly in your browser. Enter three known values and our proportion calculator instantly computes the fourth using precise cross-multiplication.
Secure Proportion Calculator Online
All proportion calculations happen locally in your browser — your numbers never leave your device. Use our proportion calculator with complete confidence and 100% privacy.
Proportion Calculator Online - No Installation
Open our proportion calculator in any browser and start solving immediately. No downloads, plugins, or accounts needed — just fast, accurate proportion solving from any device.
Solve Any Variable — Four Modes in One
Leave A, B, C, or D blank and our proportion calculator detects which unknown to solve for automatically. One tool covers all four cases with full cross-verification.
Common Use Cases for Proportion Calculator
Recipe & Cooking Scaling
Scale recipe ingredients up or down with our proportion calculator. If a recipe for 4 servings needs 3 cups of flour, solve the proportion to find exactly how much is needed for 10 servings — no guesswork required.
Map & Blueprint Distance
Convert map distances to real-world measurements using our proportion calculator. Enter the map scale ratio and the measured distance to instantly calculate the actual distance or dimensions on a blueprint.
Unit Conversion & Currency
Convert between units or currencies by setting up a known rate as a proportion. Our proportion calculator applies the same ratio to any quantity, making length, weight, volume, and currency conversions effortless.
Academic & Science Problems
Students and scientists use our proportion calculator for chemistry dilutions, physics ratios, and math homework. Solve word problems involving rates, speeds, and densities with a single calculation.
Business & Finance Ratios
Solve financial ratios and business metrics with our proportion calculator. Calculate cost-per-unit at different quantities, find break-even proportions, or apply tax rates proportionally across different revenue figures.
Design & Photography Scaling
Maintain aspect ratios when resizing images, thumbnails, or print designs using our proportion calculator. Enter the original dimensions and one new dimension to instantly find the other while preserving the correct ratio.
Understanding Proportion Calculations
What is a Proportion?
A proportion is a mathematical statement that two ratios are equal: A / B = C / D. It expresses the relationship between four quantities such that the first pair (A and B) has the same ratio as the second pair (C and D). Proportions are the foundation of everyday math — from scaling recipes to reading maps to converting units. Our proportion calculator solves for any one of the four unknowns when the other three are known, using the principle of cross-multiplication (A × D = B × C).
How Our Proportion Calculator Works
- Enter Three Known Values: Input any three of the four values (A, B, C, D) into the proportion calculator. Leave exactly one field blank — that is the unknown the tool will solve for. The calculator automatically detects which variable to solve for based on which field is empty.
- Instant Browser-Based Solving: Click Calculate and our proportion calculator applies cross-multiplication (A × D = B × C) to solve for the missing value in milliseconds. All computation runs locally in your browser — your numbers are never sent to any server.
- View and Use Your Results: The proportion calculator displays the solved value, the complete proportion equation (A / B = C / D), the C/D ratio expressed as a percentage, and a verification checkmark confirming the proportion is mathematically correct.
Proportion Formulas Explained
- Solving for A: A = (B × C) / D — derived from A × D = B × C, isolate A by dividing both sides by D.
- Solving for B: B = (A × D) / C — derived from A × D = B × C, isolate B by dividing both sides by C.
- Solving for C: C = (A × D) / B — derived from A × D = B × C, isolate C by dividing both sides by B.
- Solving for D: D = (B × C) / A — derived from A × D = B × C, isolate D by dividing both sides by A.
Proportions vs. Ratios vs. Percentages
A ratio compares two quantities (e.g., 3:4), while a proportion states that two ratios are equal (3/4 = 9/12). A percentageis a special proportion where the second ratio always has a denominator of 100 (e.g., 25% = 25/100). Our proportion calculator bridges all three: it solves the general A/B = C/D form and also displays C/D as a percentage so you can interpret the result in the most meaningful way for your use case. The "Percentage of a Whole" mode lets you directly find what percentage one number is of another, or find the whole from a percentage.
Related Tools
Voice Recorder & Audio Extractor
Record high-quality audio from your microphone or extract audio from MP4 and WebM video files offline. 100% secure, browser-based utility.
Audio Slicer & Converter
Trim audio clips and convert between MP3, WAV format client-side - Free online audio cutter
ID3 Tag & Metadata Editor
Read and write ID3 tags, album art, artist, and track details directly to MP3 file headers - Free online ID3 tag editor
MP3 Metadata Viewer
View all ID3 tags — title, artist, album, artwork, BPM, and every embedded frame — from any MP3 file instantly in your browser - Free online MP3 metadata viewer
Frequently Asked Questions About Proportion Calculator
A proportion calculator solves for an unknown value in a proportion equation A / B = C / D. When three of the four values are known, it uses cross-multiplication (A × D = B × C) to find the missing one. Our online proportion calculator detects which value is missing automatically and solves it instantly — no signup required.
To solve a proportion A / B = C / D: first cross-multiply to get A × D = B × C. Then isolate the unknown by dividing both sides by the coefficient next to it. For example, if D is unknown: D = (B × C) / A. Our proportion calculator does all of this automatically — just enter three values and click Calculate.
Cross-multiplication is the technique of multiplying the numerator of each ratio by the denominator of the other: A × D = B × C. It works because if two fractions are equal (A/B = C/D), multiplying both sides by B × D gives A × D = B × C — eliminating all denominators. This makes it straightforward to isolate any one of the four variables.
Yes! Our proportion calculator can solve for any of the four values. Simply leave one field blank and fill in the other three. The tool automatically detects which value to solve for and applies the appropriate cross-multiplication formula to find the answer.
Switch to the "Percentage of a Whole" tab to find what percentage one number is of another, or to find the whole from a percentage and a part. For example, enter Part = 25 and Whole = 80 to get 31.25%. Or enter Percentage = 25 and Whole = 80 to get Part = 20. All three modes are supported.
Yes! Our proportion calculator is 100% free with no signup, no subscription, and no usage limits. Solve as many proportions as you need directly in your browser at no cost.
Absolutely. All proportion calculations are performed locally in your browser using JavaScript. No values are transmitted to any server, and no data is stored. Your inputs and results remain completely private on your device.
Proportions appear everywhere: scaling recipes (3 eggs for 12 cookies → ? eggs for 20 cookies), map reading (1 cm = 50 km → 3 cm = ? km), unit conversion (1 inch = 2.54 cm → ? inches = 10 cm), and photography (original 1920×1080 → new width 1280 → new height ?). Our proportion calculator handles all these cases instantly.
Yes! A percentage is a proportion where one denominator is 100. For example, "25% of 80" is the same as 25/100 = ?/80. Enter A=25, B=100, D=80, leave C blank, and our proportion calculator instantly returns C=20. You can also use the dedicated "Percentage of a Whole" tab for direct percentage calculations.