This is a guide of what a wall and railing would be using a standard UK brick. The railings are based on fixed space type with 100mm gaps between bars with 12mm uprights. This might help if you are starting with a blank canvas and thinking of having a brick and iron boundary fence.
The brick piers as drawn are single brick type and have a dimension of 215mm. meaning the overall dimension of the first drawing including brick piers would roughly 1340mm in total. Single brick type piers are ok for short railings, but for taller railings a brick and a half pier would be recommended. Brick and a half piers are roughly 330mm in dimensions. this would make the first drawing 1570mm roughly in size. If you intend to have a gate within a wall it is highly recommended be of the brick and a half type to help support the weight of the gate.
These are all based on a standard 215mm uk brick with a nominal 10mm mortar joint. Actual size will vary depend on brick layer, as some seem to favour larger brick joints. This would increase the size of the railing slightly. What you might notice is the sizes commonly advertised on the internet are not useful sizes.
This does allow you to see the upright bar count, based on wall size. It is a good basis for designing you own railing design pattern.

4 full brick span between brick piers. 8 bars
imperial size Almost 3ft

4 and half brick span between brick piers. Bar count is 9.
Imperial size 3ft 4inches

5 brick span between brick piers. Bar count 10
imperial size 3ft 8 and half inches

5 and half brick span between piers bar count 11
Metric size 1247mm, imperial size 4ft 1 inch

6 brick span between piers
Metric size 1360mm, Imperial size 4ft 5 and half inches

6 and half brick span
Metric size 1472mm, Imperial size 4ft 10 inches

7 brick span
Imperial size 5ft 2 and half inches

7 and half brick span
Imperial size 5ft 7 inches

8 brick span
Imperial size 5ft 11 inches

8 and half brick span
Imperial size 6ft 4 inches

9 brick
Imperial size 6ft 8 inches

9 and half
Imperial size 7ft





