From a side
Side a
A = na²/[4 tan(π/n)]; P = na
Open calculator ↗GEOMETRY, ONE FIGURE AT A TIMERegular octagon.
Regular octagon.
Understand every measure.
Eight sides, one consistent scale. Four free tools, clear formulas and a little discovery along the way.
Find your calculator ↗FIGURE 152D / REFERENCE DRAWING
REGULAR OCTAGONNOT TO SCALE
THE CALCULATOR DESK
Start with what you know.
4 calculators available
From circumradius
Circumradius R
a = 2R sin(π/n)
Open calculator ↗From the apothem
Apothem r
a = 2r tan(π/n)
Open calculator ↗From area
Area A
a = √[4A tan(π/n)/n]
Open calculator ↗MEET THE FIGURE
A regular octagon, explained.
A regular octagon has eight equal sides and eight equal interior angles. A stop-sign outline is a familiar visual example, though this site calculates mathematical dimensions rather than any jurisdiction’s sign standard.
Understand the properties ↗LOOK A LITTLE CLOSER
More than a formula.
Cutting four equal right-isosceles corners from a suitable square can make a regular octagon.
A general eight-sided polygon is not determined by one side. These tools apply only to regular octagons.
Explore three geometry facts ↗These tools use ideal Euclidean geometry. Every input must use the same unit. Numerical results are rounded and do not replace measurement, specialist design or safety checks. See worked examples.