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Rectangle diagonal calculator

Compute the diagonal, area and perimeter of a rectangle.

Related tools

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Square diagonal calculatorCompute the diagonal and area of a square from its side length.Rectangle calculatorEvery 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).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.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.Volume calculatorCompute the volume of a box, cube, sphere, cylinder or cone.

Need Diagonal, Area, Perimeter? The Rectangle diagonal calculator derives it from Width, Height in one step. For instance, with Width = 3 and Height = 4 it returns Diagonal = 5, Area = 12 and Perimeter = 14.

How to use it

  1. Enter your values: Width, Height.
  2. Read the result instantly: Diagonal, Area, Perimeter.

Frequently asked questions

How does the Rectangle diagonal calculator work?

It takes Width and Height and derives Diagonal, Area and Perimeter from them. The calculation is live as you type, so the result updates on every change.

Which values does the calculator ask for?

2 values: Width and Height. Nothing else is required — no account, no file upload.

What does a typical calculation look like?

With Width = 3 and Height = 4, the calculator returns Diagonal = 5, Area = 12 and Perimeter = 14. 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 Width = 6 and Height = 8 instead, Diagonal goes from 5 to 10 — which is why it is worth testing a few scenarios rather than trusting a single figure.

What does it give for smaller values?

Scaled down to Width = 2 and Height = 2, Diagonal comes out at 2.828. 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 diagonal calculator and the Square diagonal calculator?

Both return Diagonal and Area. What differs is what they ask for: this one wants Width and Height, the Square diagonal calculator wants Side length. Use whichever matches the numbers you already have.

Is there a tool for the next step?

Rectangle calculator is the closest one after this: 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).

What else is worth having open alongside it?

Golden Rectangle Calculator and Aquarium volume calculator — they come up in the same task often enough to be worth a second tab.

Further reading

All guides →
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-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.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 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-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.