Surface area calculator for cubes, spheres, cylinders, cones, and more. Shows the formula for each shape and runs entirely in your browser with no signup.
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Surface area calculator for cubes, spheres, cylinders, cones, and more. Shows the formula for each shape and runs entirely in your browser with no signup.
A cone with r 3 and h 4 has a surface area of 75.40 square units.
Transparent methodology
How this calculator works
Reviewed August 2026 · BoringToolsKit Editorial Team
BoringToolsKit. “Surface Area Calculator.” boringtoolskit.com/surface-area-calculator/ (reviewed August 2026). Free to reference in articles, syllabi, and answer posts with a link.
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Worked example: a cone with radius 3 and height 4
Take r = 3, h = 4. First find the slant height: s = sqrt(3^2 + 4^2) = sqrt(25) = 5. The calculator then computes base area pi * r^2 = pi * 9 = 28.27 and lateral area pi * r * s = pi * 15 = 47.12. Adding them gives 28.27 + 47.12 = 75.40 square units, which is 24pi rounded to 2 decimals.
The same numbers show where errors usually happen. If you forget the slant height and type the vertical height into the radius field of a cylinder-style formula, you get 2 * pi * 9 + 2 * pi * 3 * 4 = 131.95, an overstated answer about 75 percent too high. Because every shape shares the same a/b/c fields, check the shape selector before trusting any output. A cube with a = 5 gives 6 * 25 = 150, and a box 2 x 3 x 4 gives 2 * (6 + 8 + 12) = 52; both land exactly on whole numbers, which makes them easy hand checks.
What surface area numbers hide in practice
The calculator computes total surface area of ideal shapes, so real materials need a margin. A cylinder r = 2, h = 5 scores 28pi, about 87.96 square units. Paint coverage of roughly 100 square feet per gallon assumes smooth sealed surfaces; corrugated metal adds 10 to 15 percent, and the instructions on a 1-gallon can of rust-preventive paint typically list 80 to 120 square feet, so a single coat of that cylinder in feet-scale needs closer to 1.1 gallons than 0.88.
Two units rules matter. All three inputs must share one unit before you click: mixing a radius in inches with a height in feet silently inflates the result by 12x. And when a hollow object only needs exterior coverage, subtract the base or lid you will not paint; for an open-top tank r = 2, h = 5, dropping one circular base removes 12.57, about 14 percent of the total. Curved shapes also ignore seams and overlap, which add another 5 to 10 percent on wraps of pipe or ductwork.
