Calculate the length of a rectangle from area and width, perimeter and width, or diagonal and width. Get all rectangle properties instantly.
The Length of a Rectangle Calculator helps you find the missing length of a rectangle when you know certain other measurements. Whether you have the area and width, the perimeter and width, or the diagonal and width, this tool instantly computes the length and all related rectangle properties.
Rectangles are among the most common shapes in everyday life — from screens and papers to rooms and playing fields. Being able to quickly determine a rectangle's length from partial measurements is essential in architecture, engineering, carpentry, interior design, and academic math courses.
This calculator supports three solve modes. In "Area + Width" mode, it divides the area by the width to find the length. In "Perimeter + Width" mode, it subtracts twice the width from the perimeter and divides by two. In "Diagonal + Width" mode, it uses the Pythagorean theorem to derive the length from the diagonal and width.
Beyond just the length, the tool computes the full set of rectangle properties: area, perimeter, diagonal, aspect ratio, and circumradius. Visual comparison bars let you see how length and width relate, and a reference table of common real-world rectangles provides useful context. Eight presets cover typical problems so you can explore results immediately.
Length of a Rectangle problems often require several dependent steps, and a small arithmetic slip can propagate through every derived value. This calculator is tailored to that workflow: you enter width, decimal places, solve mode, and it returns length, width, area, perimeter in one consistent pass. It is useful for homework checks, worksheet generation, tutoring walkthroughs, and fast field/design estimates where you need reliable geometry results without rebuilding the full derivation each time.
From area: L = A ÷ W | From perimeter: L = (P − 2W) ÷ 2 | From diagonal: L = √(d² − W²)
Result: Length = 15
Dividing the area (120) by the width (8) gives a length of 15. The perimeter is 2×(15+8) = 46, and the diagonal is √(225+64) ≈ 17.
This length of a rectangle tool links the entered values (width, decimal places, solve mode, unit) to the target geometry relationships used in class and practice problems. Instead of solving each intermediate step manually, you can validate setup and arithmetic quickly while still tracing which measurements drive the final result.
Formula focus: the calculator formula
Length of a Rectangle shows up in school geometry, technical drafting, construction layout checks, and early engineering design estimates. When values are changed repeatedly, the calculator helps you compare scenarios quickly and see how sensitive the shape is to each dimension.
Start with the primary outputs (length, width, area, perimeter) and then use the remaining cards/tables to confirm consistency with your diagram. Keep units consistent across inputs, and round only at the end if your assignment or project specifies a fixed precision.
Divide the area by the width: Length = Area ÷ Width. For example, if the area is 120 cm² and the width is 8 cm, the length is 15 cm.
You need the perimeter and the width. Then Length = (Perimeter − 2 × Width) ÷ 2.
Use the Pythagorean theorem: Length = √(Diagonal² − Width²). The diagonal must be longer than the width.
The aspect ratio is the length divided by the width. A square has an aspect ratio of 1:1, while a widescreen display is 16:9.
The circumradius is half the diagonal — it is the radius of the circle that passes through all four corners of the rectangle. Use this as a practical reminder before finalizing the result.
Yes. A square is a special rectangle where length equals width. All formulas apply identically.