How the calculator works
Perimeter is the distance all the way round, so it’s just the three sides added together: P = a + b + c.
Sides of 6 cm, 9 cm and 11 cm give 6 + 9 + 11 = 26 cm. The answer stays in plain centimetres, not cm², because you’re measuring a length, not a surface.
Before adding anything up, the calculator checks your three lengths can close into a shape. If they can’t, it tells you instead of handing back a number for a triangle that doesn’t exist.
Will your three lengths make a triangle?
Put the sides in order. If the two shortest add up to more than the longest, you have a triangle. That one comparison is enough, because every other pair already includes the longest side.
Take 4, 6 and 8 m: 4 + 6 = 10, which beats 8, so it works. Try 2, 3 and 6 m: 2 + 3 = 5, which falls short of 6, so the two short sides can’t reach each other. A tie, such as 3, 4 and 7, doesn’t count either. The short sides lie flat along the long one and you get a straight line with no area.
Reading the other results
Type. Three matching sides make an equilateral triangle. If exactly two match, or all three do, it also counts as isosceles. When no two sides are the same, it’s scalene. This is based on sides only, not angles.
Semiperimeter. Half the perimeter, written s. For the 6–9–11 triangle, s = 13 cm. On its own it isn’t very useful, but the area calculation depends on it.
Area. With all three sides known, the calculator uses Heron’s formula: A = √(s(s − a)(s − b)(s − c)). For 6–9–11 that’s √(13 × 7 × 4 × 2) = √728 ≈ 26.98 cm². No height measurement needed.
Buying edging, fencing or trim
A triangular bed with sides of 5 m, 7 m and 8 m has a 20 m perimeter. That’s the bare geometric figure, so add extra for gates, overlaps at joins and offcuts before you order.
The same goes for a 40–50–60 cm panel. Its 150 cm perimeter is the exact edge length, but mitred corners eat into a length of moulding, so buy more than the perimeter suggests.