How Rule of Three calculadora solves the problem

Think of Rule of Three calculadora as the back-of-the-envelope version of the calculation, only the envelope is a web page and the arithmetic is audited by our test suite.

This is the kind of problem where a stray decimal costs you the mark. Think of one worked example you can reuse — then crunch the numbers and the rest of this page explains what the answer means.

Classic cross-multiplication. If 3 kg of flour costs £2.10, 7 kg costs (7 × 2.10) / 3 = £4.90. Works whenever the relationship between two quantities is strictly proportional.

The formula we run is If a : b = c : x then x = (b × c) / a. You'll see each term laid out in the worked example below.

Seeing it on real numbers

A working example keeps the formula honest:

Classic cross-multiplication. If 3 kg of flour costs £2.10, 7 kg costs (7 × 2.10) / 3 = £4.90. Works whenever the relationship between two quantities is strictly proportional.

Every run comes back to If a : b = c : x then x = (b × c) / a — change the inputs, the structure of the answer stays.

Scenarios where Rule of Three calculadora pays off

Rule of Three calculadora is aimed at people arriving with questions like these:

"Rule of three"
"Direct proportion"
"Inverse proportion"
"Cross multiplication"
"What is rule of three"
"How to calculate rule of three"

When it isn't the right tool

Every tool has an edge where it stops being the right answer. Rule of Three calculadora is no exception:

For legally binding tax or medical decisions — cross-check with HMRC, NHS or a qualified professional.
For very large or very small extremes the rounding error outgrows the useful precision.
When the underlying rate or threshold has changed since the page was last reviewed — always verify with the primary source.
When the input you have is already a derived figure (net of something) — feeding it in as "gross" will double-subtract.

Traps to steer around

Every time you crunch the numbers for a new scenario, one of these creeps in — it's worth knowing them ahead of time.

Mixing up units — grams in one field, ounces in another, then wondering why the answer is off.
Treating a percentage as a whole number. 20% means 0.20 in the maths, not 20.
Rounding at every step. Keep four decimals internally and only round the final number.
Using last year's thresholds. If the page isn't dated, assume it's stale and check GOV.UK.
Reading a tool like this as advice. It is maths, not a decision — the decision is still yours.

The sources behind the numbers

Where the maths needs an external authority, we cross-check against:

BBC Bitesize
MathsIsFun

Works well alongside

If this question keeps coming up for you, the same cluster of tools usually comes next:

Percentage calculadora — Work out a percentage of a value, the percentage between two values, and percentage increases or decreases — with the formula shown.
Weighted Average calculadora — Compute a weighted mean where each value carries a different weight — ideal for coursework grades, portfolio returns and sample averages.

How we keep this accurate

Our calculadoras run on pure, unit-tested functions — the same logic lives in the browser and in the CI test suite. When tax rates, thresholds or official figures move, the update lands within 24 hours of the announcement. You can read the editorial policy and corrections policy.

Found an out-of-date number on Rule of Three calculadora or anywhere else in the Maths toolkit? Send it to the editorial desk and we'll patch it. Or browse the full calculadora directory for the next tool you need.

Frequently asked questions

Rule of three?

Put simply, feed the figures into the Rule of Three calculadora widget and it'll show the working. Solve a proportion by cross-multiplication — the classic rule-of-three used for recipe scaling, unit pricing and percent conversions. Classic cross-multiplication. If 3 kg of flour costs £2.10, 7 kg costs (7 × 2.10) / 3 = £4.90. Works whenever the relationship between two quantities is strictly proportional.

Direct proportion?

Short answer: the underlying formula is **If a : b = c : x then x = (b × c) / a**. Classic cross-multiplication. If 3 kg of flour costs £2.10, 7 kg costs (7 × 2.10) / 3 = £4.90. Works whenever the relationship between two quantities is strictly proportional.

Inverse proportion?

Quick version: this question usually arrives alongside Percentage calculadora, Weighted Average calculadora. The Rule of Three calculadora handles the specific case above; the others cover adjacent ground.

Cross multiplication?

Practically speaking, every figure is cross-checked against BBC Bitesize and the wider data. If you notice a stale rate, email the editorial desk and we'll patch it in under 24 hours.

What is rule of three?

Here's the plain-English summary: yes, everything runs in your browser. No inputs are sent to our servers or any third party, nothing is logged and nothing persists after you close the tab.

How to calculate rule of three?

In one line: Rule of Three calculadora is free to use, free to share and free to embed — pass the URL around a class, a slack channel or a family chat. The editorial policy covers attribution.

Rule of three formula?

Put simply, the short method: write the inputs in the units shown, run the calculation, then sense-check the answer against an order-of-magnitude estimate in your head.

Rule of three example?

Short answer: if the result surprises you, run it a second time with slightly different inputs — small swings often reveal a unit or rounding issue in the original figures.

Rule of three worked example?

Quick version: a calculadora is a sanity check, not a verdict. For anything legally binding — contracts, tax filings, medical decisions — bring the figure to a qualified professional as a starting point.

Rule of three explained?

Practically speaking, Solve a proportion by cross-multiplication — the classic rule-of-three used for recipe scaling, unit pricing and percent conversions. The page walks through the method in full so you can answer follow-up questions without guessing.

Rule of three definition?

Here's the plain-English summary: open the Rule of Three calculadora widget at the top of the page. Solve a proportion by cross-multiplication — the classic rule-of-three used for recipe scaling, unit pricing and percent conversions. Classic cross-multiplication. If 3 kg of flour costs £2.10, 7 kg costs (7 × 2.10) / 3 = £4.90. Works whenever the relationship between two quantities is strictly proportional.

Rule of three meaning?

In one line: open the Rule of Three calculadora widget at the top of the page. Solve a proportion by cross-multiplication — the classic rule-of-three used for recipe scaling, unit pricing and percent conversions. Classic cross-multiplication. If 3 kg of flour costs £2.10, 7 kg costs (7 × 2.10) / 3 = £4.90. Works whenever the relationship between two quantities is strictly proportional.

How it works