Work out proportions in seconds with this clean, professional calculator. Enter any three values in a:b = c:d and it will find the fourth using cross-multiplication, then simplify the ratio and show clear working steps. You can also check whether two ratios form a true proportion, and scale a ratio to hit a specific total (e.g., split a budget or recipe). The tool accepts integers, fractions, decimals and percentages, making it ideal for schoolwork, teaching, recipes, construction, maps and finance. It’s fast, mobile-friendly and keyboard-ready, with optional focus and fullscreen modes for distraction-free use.
: or /, decimals or % are fine).Q: What is a proportion?
A proportion states two ratios are equal, e.g., a:b = c:d.
Q: How do I find a missing value in a proportion?
Use cross-multiplication. Example: a:b = c:d → a × d = b × c. Solve for the unknown:
Q: What’s the difference between a ratio and a proportion?
A ratio compares two quantities (a:b). A proportion is an equation showing two ratios are equal (a:b = c:d).
Q: Can I use fractions, decimals and percentages?
Yes—enter 3/4, 0.75 or 75%. The calculator converts and simplifies automatically.
Q: How do I scale a ratio to a target total?
Add the parts (for 2:3 total is 5), divide the target by total parts (350 ÷ 5 = 70), then multiply each part: 2×70 and 3×70.
Q: What is a unit rate and scale factor?
A unit rate is a ratio per one unit (e.g., £12 per kg). A scale factor is what you multiply by to grow/shrink a ratio or figure (maps, drawings, recipes).
Q: Does this handle inverse proportion?
This tool focuses on direct proportion. For inverse proportion (x ∝ 1/y), use a dedicated inverse/variation calculator page.