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Rectangle calculator

Every property of a rectangle from any two you know. Pick what you have — length & width, area & a side, perimeter & a side, or diagonal & a side — and it returns the length, width, area, perimeter and diagonal, using A = l·w, P = 2(l+w) and d = √(l²+w²).

Related tools

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Rectangle diagonal calculatorCompute the diagonal, area and perimeter of a rectangle.Golden Rectangle CalculatorFrom the long side, short side, diagonal, area or perimeter, derive the whole golden rectangle — and see it drawn with its golden-spiral subdivision.Regular polygon calculatorEvery measure of a regular polygon from the number of sides and the side length: the area, perimeter, interior and exterior angles, the sum of interior angles, the apothem (inradius), the circumradius and the number of diagonals. One entry, the full geometry — for triangles through many-sided polygons.Square calculatorAll four measures of a square from any one. Give the side, the area, the perimeter or the diagonal and it returns the rest — side s, area s², perimeter 4s and diagonal s√2 — instantly.Aquarium volume calculatorCompute the water volume of a rectangular aquarium in litres and gallons.Area calculatorCompute the area of a rectangle, square, circle or triangle.Equilateral triangle calculatorCompute the area, height and perimeter of an equilateral triangle from its side.Perimeter calculatorCompute the perimeter of a rectangle, square, circle or triangle.

The Rectangle calculator turns What do you know?, Value 1 (length / area / perimeter / diagonal), Value 2 (width or length) into Length, Width, Area, Perimeter, Diagonal, instantly and for free. For instance, with What do you know? = Length and width, Value 1 (length / area / perimeter / diagonal) = 10 and Value 2 (width or length) = 6 it returns Length = 10, Width = 6 and Area = 60.

How to use it

  1. Enter your values: What do you know?, Value 1 (length / area / perimeter / diagonal), Value 2 (width or length).
  2. Read the result instantly: Length, Width, Area, Perimeter, Diagonal.

Frequently asked questions

How does the Rectangle calculator work?

It takes What do you know?, Value 1 (length / area / perimeter / diagonal) and Value 2 (width or length) and derives Length, Width, Area, Perimeter and Diagonal from them. The calculation is live as you type, so the result updates on every change.

Which values does the calculator ask for?

3 values: What do you know?, Value 1 (length / area / perimeter / diagonal) and Value 2 (width or length). Nothing else is required — no account, no file upload.

What does a typical calculation look like?

With What do you know? = Length and width, Value 1 (length / area / perimeter / diagonal) = 10 and Value 2 (width or length) = 6, the calculator returns Length = 10, Width = 6 and Area = 60. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

How much does the result change with different inputs?

It moves a lot. Using What do you know? = Area and length, Value 1 (length / area / perimeter / diagonal) = 20 and Value 2 (width or length) = 12 instead, Length goes from 10 to 12 — which is why it is worth testing a few scenarios rather than trusting a single figure.

Which “What do you know?” option should I choose?

You can pick between « Length and width », « Area and length », « Perimeter and length » and « Diagonal and length ». Each one changes what the calculator works out, so switch and compare — the default is « Length and width ».

What does it give for smaller values?

Scaled down to What do you know? = Length and width, Value 1 (length / area / perimeter / diagonal) = 5 and Value 2 (width or length) = 3, Length comes out at 5. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

Homework and site work alike: how much paint a wall takes, how much water a tank holds, whether a corner is square — and, on the coordinate side, the slope between two points, the distance across a plan, the midpoint of a span.

What is the most common mistake?

Mixing units inside one shape. Entering a length in centimetres and a width in metres gives an area wrong by a factor of a hundred, and the result looks perfectly plausible.

What is the difference between the Rectangle calculator and the Rectangle diagonal calculator?

Both return Area and Perimeter. What differs is what they ask for: this one wants What do you know? and Value 1 (length / area / perimeter / diagonal), the Rectangle diagonal calculator wants Width and Height. Use whichever matches the numbers you already have.

Is there a tool for the next step?

Golden Rectangle Calculator is the closest one after this: From the long side, short side, diagonal, area or perimeter, derive the whole golden rectangle — and see it drawn with its golden-spiral subdivision.

Further reading

All guides →
ExplainerThe Pythagorean Theorem ExplainedIn a right triangle, a² + b² = c². Here's what the formula means, a worked example, how to find any side, and where it's used in real life.How-toHow to Calculate VolumeVolume is the space inside a shape. Here's how to find it for boxes, cylinders and spheres, and why keeping your units consistent is the whole game.How-toHow to Calculate the Square Metres of a RoomFind a room's area by multiplying length by width. Here's the method for rectangular, L-shaped and round rooms, and why area drives your material orders.How-toCircle: Circumference and AreaCircumference is 2πr, area is πr². Here's how radius, diameter and π fit together, both formulas worked through, and how not to mix them up.How-toHow Much Substrate Does an Aquarium Need? Depth, Weight and Grain SizeThe depth follows the job: a bare display, a planted tank whose roots need room, an under-gravel plate. Then comes the number people are unprepared for — substrate is what puts a filled tank past what a piece of furniture can hold. Plus why fine sand compacts and coarse gravel traps waste.ExplainerHow Many Fish Can a Tank Hold? The Inch-per-Gallon Rule Is WrongEvery stocking calculator implements a length-per-volume rule, and waste output scales with body mass, not length. A fish twice as long makes nearly five times the ammonia. Run through the tool's five methods, the same tank comes back holding anywhere from 6 to 12 fish.