The calculation, without hidden assumptions
See the core rectangle measurements together: area, perimeter, diagonal and aspect ratio from a single length and width. Enter a measured diagonal as well and the calculator compares it with the theoretical diagonal, which is the classic way to check whether a foundation, deck or frame is actually square.
How to use this calculator
Enter the length and width in the same unit.
Optionally enter a diagonal you measured on site.
Review area, perimeter, diagonal, aspect ratio and the squareness difference.
Where people use it
- •Room and slab geometry
- •Checking that a deck or frame layout is square
- •Estimating the area and edging for a rectangular plot
Example: 12 × 9
Area 108, perimeter 42, diagonal 15. If you measure 15.1 between opposite corners the layout is 0.1 longer than a perfect rectangle, so one corner is slightly out of square.
What the result does not assume
- •Assumes four right angles and straight sides; irregular quadrilaterals need separate measurements.
- •Use the same unit for length, width and the measured diagonal.
Frequently asked questions
How do I calculate the area of a rectangle?+
Multiply the length by the width. The result is in square units of whatever unit you entered.
How can the diagonal show that something is square?+
In a true rectangle both diagonals equal √(length² + width²). A measured diagonal that differs noticeably from that value means the corners are not right angles.
Does the aspect ratio change the area?+
No. The ratio only describes the proportion of length to width; the area depends on the actual lengths.
Semantic next steps
Continue the calculation
These links move to a different input, formula or project stage rather than a keyword variation of this page.
Area Calculator — Rectangle, Triangle, Circle, Trapezoid & More
Calculate area for rectangle, square, triangle, circle, trapezoid, parallelogram, ellipse and sector shapes with editable units, converted output units and the substituted formula shown.
OpenPerimeter Calculator — Rectangle, Square, Triangle & Circle
Calculate the perimeter of a rectangle, square or triangle, or the circumference of a circle, with the substituted working.
OpenSquare Footage Calculator
Calculate square footage for rectangles, circles and triangles, convert the area to square meters and estimate cost from a price per square foot.
OpenVolume & Cubic Yard Calculator — Shapes, yd³, ft³, m³, Weight & Cost
Calculate box, cylinder, sphere or area-by-depth volume and instantly convert the result to cubic feet, cubic yards and cubic metres, with optional waste, material density, weight and bulk cost.
OpenScale Calculator — Drawings, Maps and Models
Convert real measurements to scaled drawing or model sizes, convert scaled sizes back to real dimensions, or find the 1:N scale ratio.
OpenRectangle Diagonal Calculator — Square-Up & Pythagorean Check
Calculate the corner-to-corner diagonal of a rectangle from length and width, then optionally compare a field-measured diagonal with the theoretical value.
OpenCabinet Door Calculator — Overlay, Inset, Pair Gap & Shaker Cut List
Calculate finished cabinet door dimensions for inset or overlay openings, single or paired doors, then estimate Shaker rail, stile and panel cut sizes from your actual joinery allowances.
OpenPond Liner Calculator — Sheet Size, Area & Roll Length
Size a rectangular pond liner from pond length, width, maximum depth and edge overlap, then estimate sheet area and roll lanes from an entered roll width.
OpenMore in this calculator collection
Browse nearby calculators in the same topic cluster so related pages reinforce each other instead of sitting as isolated URLs.
Free publisher widget
Put this calculator on your website
Give readers the live calculator without rebuilding the math. Copy the responsive embed in one click, or customize width, height, border and corner radius before publishing. The branded MAXScanner attribution stays visible.
Calculator embed HTML
Paste this code into your page. Focus the code box to select it manually.
Keep the result connected to the real job
MAXScanner keeps the formula and assumptions beside the result so you can verify the number before using it in a drawing, estimate, specification, worksheet or document. Where a supplier, manufacturer, drawing or applicable standard owns a requirement, that source remains authoritative.