The quick, real-world answer is 18 inches, measured from the ground to the top of the seat surface. That is about 46 cm. Our standard outdoor bench is built around that height because it places most adults in a natural sitting position: feet flat on the ground, knees bent at roughly a right angle, and no pressure at the back of the thighs.
If you check typical park benches and garden benches, most seat heights sit between 17 and 19 inches. Eighteen inches is the center of that comfortable range, which is why we use it as the default. For a bench with a thick seat cushion, the frame may start slightly lower so the finished seating height still lands close to 18 inches. A backless bench or a slim wooden bench can feel comfortable at 17 to 18 inches, while a deeper bench with a reclined back may work well at 18 to 20 inches.
There is also an accessibility guideline: benches intended to be accessible often follow a 17-to-19-inch seat height, with 18 inches as the target. That makes it easier for many people to sit down and stand up without needing to drop too low or climb up.
If you are measuring one of our benches or any outdoor bench, measure from the flat ground to the top of the seat board at the front edge. Do not measure to the underside of the seat. On uneven ground, measure at the location where the front legs rest, because that is where a person actually sits.
For most adults, the right range is 16 to 20 inches. Shorter users may prefer 16 to 17 inches, while taller users may feel more comfortable at 19 to 20 inches. But if you want one safe, standard height that works for public spaces, patios, gardens, decks, and entryways, 18 inches from the ground is the number we recommend and use on our standard outdoor bench.