Perimeter of a Triangle Calculator P = a + b + c. a b c Perimeter 18 Step-by-step with your numbers: 1. Values used: 2. a = 5 3. b = 6 4. c = 7 5. Perimeter = 18 Did we solve your problem today? 👍 Yes 👎 No Thanks for the feedback! Sum of all sides. FAQ Right triangle? Still a+b+c. Related calculators Triangle Area Calculator Find a triangle's area from its base and height. Heron's Formula Calculator Find a triangle's area from its three sides. Perimeter Calculator Rectangle perimeter.