Railings protect people from falling off stairs and decks. British building regulations do not cover requirements for patio balustrades, but they do cover railing for stairs and landings. If you are building a deck or a balcony, you should apply the rules on landings contained in Document K of the British building regulations. The code specifies the maximum space between each baluster as 99 mm (3.9 inches). The exact requirement is that the space does not allow a 100 mm (3.94 inches) sphere to pass between balusters. This measurement was arrived at as being roughly the size of a child's head.
Measure the distance between the two end posts supporting the top rail.
Add the width of a baluster to the maximum spacing -- 99 mm (3.9 inches). For example, for a 5 by 5 cm (2 by 2) inch baluster with a maximum 99 mm (3.9 inches), add 3.7 cm (1.5 inches) to to 9.9 cm (3.9 inches), which gives you 13.6 cm (5.4 inches). Remember, a 5 cm by 5 cm (2 by 2 inch) picket is actually 3.7 cm (1.5 inches) wide because the initial measurement is taken before the wood is planed to achieve the ideal surface.
Divide this figure into the distance between the two end posts. For example, if your distance between end posts is 180 cm (72 inches), divide 180 by 13.6 to get 13.23.
Round the figure up to get the number of required balusters. In this case the answer is 13 so round up to 14.
Multiply the number of balusters by the width of one baluster. In this case, 14 multiplied by 3.7 is 51.8 cm (20.4 inches).
Subtract the result from the total distance between two posts. For example, 180 minus 51.8 is 128.2 cm (51.25 inches).
Divide this result by the number of spaces between balusters (one more than the number of balusters, since you will have space before and after the last one). In this case, divide 128.2 by 15 (14 plus 1) to get 8.55 cm (3.42 inches). This is the measurement of the open space between each baluster.
Step back after installing each baluster to make sure the spacing is visually correct. The most common errors in calculation are to forget to add the extra space before the final division and to forget to use the baluster's actual measurement instead of the raw wood dimension.