FenceMeasure the actual product
Nominal timber names describe a size family; they are not reliable inputs for finished board width. Read the supplier specification and measure a sample when fit is critical. Record width, thickness and usable length separately. Treat rails and posts as separate products.
If a clear bay is 2.31 m, 140 mm boards and 10 mm gaps give about 16 boards. Entering a nominal 150 mm width changes the quantity and may leave the ordering list short. Board overlap has the same sensitivity because it changes effective coverage.
Keep units consistent. Convert millimetres to metres when entering the metric tool: 140 mm = 0.14 m. In feet mode, enter decimal feet; 5.5 inches = 0.458333 feet. Do not assume that a rail label equals installed span or that cut leftovers can always be reused.