Why your three side lengths do not make a triangle
Three positive side measurements alone do not guarantee a triangle. The longest side must be shorter than the other two combined.
Triangle calculator →Use the longest-side shortcut
Sort the lengths and compare the largest with the sum of the other two. For 3, 4 and 8, 3+4 is only 7, so the ends cannot meet. For 3, 4 and 7, they meet only on a line, with no enclosed area. For 3, 4 and 5, they enclose a valid triangle with area 6 and perimeter 12.
Look for a unit or copying mistake
An apparent impossibility often comes from mixing centimetres and metres, mistaking a diameter for a side, or including a diagonal from another shape. Convert every measurement to the same unit before entry. Do not “fix” a rejected length by subtracting a small amount merely to get an answer; the changed number now describes a different object.
Near-flat triangles need care
Even valid lengths close to the boundary can yield a very small area. Sides 1, 1 and 1.999 describe a thin triangle whose area is about 0.031603. Small measurement errors then cause large relative area changes. The calculator can evaluate the entered numbers, but it cannot determine whether your measuring instrument distinguishes a thin triangle from an almost straight line. Recheck the actual dimensions before using such results for materials or land measurements.
Try the calculators
- Triangle calculator
Calculate area and perimeter from three sides.
- Rectangle area
Calculate a rectangle’s area and perimeter.
- Scientific calculator
Arithmetic, powers, trigonometry and logarithms.