Surface Area Calculator
Calculate the total surface area of a rectangular box (all six sides) from its length, width, and height.
Surface area is the total area of every outside face of a box added together. It’s the number you need whenever material has to cover the entire outside of something — not just fill the inside of it.
How this calculator works
A rectangular box has six flat faces, arranged in three matching pairs: top and bottom, front and back, left and right. Each pair shares the same dimensions, so the calculator finds the area of one face from each pair, adds the three areas together, and doubles the result to account for both faces in each pair.
Formula: Surface area = 2 × (length × width + length × height + width × height) — the sum of all six rectangular faces.
Worked example
A box 8 ft long, 5 ft wide, and 3 ft tall:
- Length × width: 8 × 5 = 40
- Length × height: 8 × 3 = 24
- Width × height: 5 × 3 = 15
- Sum: 40 + 24 + 15 = 79
- Surface area: 2 × 79 = 158 sq ft
That’s the combined area of all six sides of the box.
Notes
This calculator covers a rectangular box or prism, which is the most common shape in construction — think of a foundation wall, a raised planter, or a rectangular tank. The formula only works when every face is a flat rectangle.
For a curved shape like a cylinder or a sphere, surface area follows a completely different formula, since those shapes don’t have flat rectangular faces. Those shapes need their own dedicated calculation.
If what you actually need is how much a box-shaped space can hold rather than how much its outside covers, that’s volume, not surface area — the site’s other volume calculators handle that question for cylinders, spheres, and rectangular spaces.
How to use
Measure the length, width, and height of the box in the same unit (typically feet). Enter all three, and the calculator returns the combined area of all six faces. Use that total when ordering material meant to cover the entire outside of the box — coating, cladding, insulation wrap, or paint.
A pure geometry calculation. Always double-check real-world measurements before ordering material.
Frequently asked questions
What's the formula for the surface area of a box?
Surface area = 2 × (length × width + length × height + width × height). A rectangular box has six faces arranged in three matching pairs — top/bottom, front/back, and left/right — so you find the area of one face from each pair, add them, then double the total.
How is surface area different from volume?
Surface area is the total area of all the outside faces of a shape, measured in square units like square feet — it's what you'd paint or wrap. Volume is the space inside the shape, measured in cubic units like cubic feet — it's what you'd fill. They use different formulas and answer different questions.
What projects use total surface area?
Anywhere you need to cover or clad every outside face of a box-shaped object — coating a foundation wall on all sides, wrapping a tank in insulation, painting a storage box, or estimating material for a planter or raised bed with a closed bottom.
Does this work for a shape that isn't a rectangular box?
No — this formula only applies to a rectangular box or prism, where every face is a rectangle. Curved shapes like cylinders and spheres use different surface area formulas since their faces aren't flat rectangles.
Estimates only. Verify quantities with your supplier before purchasing.