Simple Rule of Three Calculator

Solve mathematical proportions instantly. The definitive tool for calculating quantities, recipes, discounts and conversions.

Initial quantity
Is to
Known value
New quantity
Will be
Calculated value
(B × C) ÷ A = X
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Frequently Asked Questions

What is the simple direct rule of three?

It is a mathematical method for solving proportionality problems between three known values and one unknown. It is called 'direct' because when one quantity increases, the other increases in the same proportion.

When should I use the inverse rule of three?

It is used when the quantities are inversely proportional (if one increases, the other decreases). For example: if 2 painters take 5 hours, 4 painters will take less time. Our current calculator focuses on direct proportionality.

Is it accurate for professional calculations?

Yes. Our calculator uses high-fidelity floating point precision, ideal for dosage adjustments, graphic design scales, commercial budgets and recipe conversions.

How is it calculated manually?

The formula is: (Value B * Value C) / Value A = X. That is, you multiply the diagonal values and divide the result by the remaining value.

# What is the rule of three used for?

The simple direct rule of three is the "Swiss army knife" of mathematics. It solves problems where three data points are known and we seek the fourth. If you know the relationship between A and B, and you have a new value C, the calculator finds X instantly.

Cooking and Recipes

If the recipe calls for 500g of flour for 4 people, how much do I need for 7?

  • (500 × 7) ÷ 4 = 875g

Shopping and Prices

If a pack of 3 cans costs €2.50, how much would 10 cans cost me?

  • (2.50 × 10) ÷ 3 = €8.33

Travel and Distance

If I take 45 min to do 60km, how long will it take me to do 140km?

  • (45 × 140) ÷ 60 = 105 min

Design and Screens

If the image is 1920px wide and I want to reduce it to 800px maintaining the proportion...

  • (OriginalHeight × 800) ÷ 1920 = NewHeight

# How does the formula work?

The formula is always the same: multiply the diagonal values and divide by the remaining one.
    

            
(B × C) ÷ A = X
In the calculator: we multiply value B by value C and divide by A. The result is X.

# Professional and everyday uses

Beyond school mathematics, the rule of three is everywhere: in pharmacy for calculating dosages by body weight, in photography for calculating exposure times, in finance for calculating percentage variations, or in printing for scaling format dimensions.
Precision tip
The result is displayed with up to 4 significant decimal places. If you work with money or exact measurements, remember to round according to context: euros to 2 decimal places, pixels to integers.

Bibliographic References

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