Calculateur de ratio

Ratio Calculator

A ratio expresses the relationship between two or more quantities — how much of one thing there is compared to another. Ratios appear everywhere: recipe ingredient amounts, map scales, financial metrics, screen dimensions, and concrete mixing guides. This calculator simplifies ratios to their lowest terms, solves proportions with a missing value, and scales ratios up or down to a target total. Enter your values and get results in ratio, fraction, and percentage forms.

How Ratios and Proportions Work

Simplifying a ratio: Divide all parts by their greatest common divisor (GCD). The GCD of 18 and 24 is 6, so 18:24 simplifies to 3:4.

Scaling a ratio to a total: Add the ratio parts to find the number of units. Then: value of one unit = total / sum of parts. Multiply each part by the unit value.

Solving a proportion (finding a missing value): A proportion states two ratios are equal: A/B = C/D. Cross-multiply to solve: A x D = B x C. If D is unknown: D = (B x C) / A.

Ratio to percentage: Express the part as a fraction of the whole. For ratio 3:5, the first part is 3/(3+5) = 3/8 = 37.5% and the second is 5/8 = 62.5%.

Worked Examples

Example 1 — Simplifying a paint mixture ratio. A recipe calls for 36 parts blue to 48 parts white. GCD(36, 48) = 12. Simplified: 36/12 : 48/12 = 3:4. Every future batch just needs 3 units of blue for every 4 units of white, regardless of total volume.

Example 2 — Splitting a profit in a 5:3 ratio. Two partners split $12,400 profit in a 5:3 ratio. Total parts: 5 + 3 = 8. Value of one part: $12,400 / 8 = $1,550. Partner A: 5 x $1,550 = $7,750. Partner B: 3 x $1,550 = $4,650. Check: $7,750 + $4,650 = $12,400.

Example 3 — Scaling a recipe using a proportion. A cookie recipe for 24 cookies uses 2.5 cups of flour. You want to make 60 cookies. Set up the proportion: 2.5/24 = x/60. Cross-multiply: 2.5 x 60 = 24 x x. 150 = 24x. x = 150 / 24 = 6.25 cups of flour. Apply the same scale factor (60/24 = 2.5) to every other ingredient.

Ratio Reference Table

When to Use This Calculator

• Recipe scaling — find the exact ingredient amounts when doubling, tripling, or halving a recipe without guessing
• Financial allocations — split budgets, profits, or costs between multiple parties in an agreed proportion
• Construction and mixing — calculate concrete, mortar, or fertilizer blend quantities from a specification like 1:2:3 (cement:sand:gravel)
• Map and scale drawings — convert between map distances and real-world distances when the scale ratio is given (e.g., 1:50,000)
• Screen and image sizing — maintain aspect ratio (16:9, 4:3, 1:1) when resizing graphics, video frames, or display output

Common Mistakes

1. Reversing the order — 3:5 and 5:3 are different ratios. When labeling a ratio (miles per gallon, wins to losses), always note which quantity is A and which is B before computing.
2. Mixing units before forming a ratio — a ratio of 2 feet to 36 inches is not 2:36; convert to the same unit first (2 feet = 24 inches, so the ratio is 24:36 = 2:3).
3. Confusing part-to-part with part-to-whole — a class with 12 girls and 8 boys has a girls-to-boys ratio of 3:2, but girls make up 3/5 (60%) of the whole class. Mixing these up gives wrong percentages.
4. Not simplifying before scaling — starting a scale calculation with 120:160 instead of 3:4 makes the arithmetic harder and more error-prone; always simplify first.

Context and Applications

Ratios underpin proportional reasoning, one of the most widely applied mathematical concepts outside a classroom. Financial analysts use price-to-earnings ratios (P/E) to compare stock valuations across companies. Engineers specify gear ratios to control output torque and speed. Pharmacists compound medications by weight ratio. Photographers frame shots using the rule of thirds, which divides the frame into a 1:1:1 grid. Ecologists measure predator-to-prey ratios to track population health. In each of these cases, the same core operation — divide both values by the GCD, scale by a common factor, or cross-multiply to find a missing term — applies directly.

Tips

• To quickly simplify a ratio mentally, keep halving both numbers until one of them is odd: 48:72 — 24:36 — 12:18 — 6:9 — 2:3
• A ratio of 1:1.618 is the golden ratio (phi); it appears in design, architecture, and nature and is considered visually balanced — useful to know if you are sizing layouts or frames
• When a specification gives a percentage split (60/40), convert to a ratio by treating the percentages as the parts: 60:40 simplifies to 3:2
• For three-part ratios like 2:3:5, add all three parts (total = 10) then distribute any total proportionally: $500 splits into $100, $150, and $250
• Screen resolutions that share a ratio are equivalent aspect ratios: 1280x720, 1920x1080, and 3840x2160 are all 16:9
• Cross-multiplication only works when the two ratios are set equal (a proportion); do not apply it to a single unsimplified ratio