Start a brick layout with the available distance and the face of the brick placed along it. Mortar also takes space. Draw the beginning, end and measurement direction before dividing the length.
Header and stretcher modules
A header shows the short face of a brick; a stretcher shows the long face. For repeating headers, add header width and one head joint. For stretchers, add brick length and one head joint. A 100 mm header plus a 10 mm joint gives a 110 mm horizontal module.
A separate row has one fewer internal joint
Ten 100 mm headers with nine 10 mm joints total 1090 mm. This is 10 × 110 − 10. Subtracting one joint applies to a row without end joints. Add a joint at either end separately. The formula does not decide which boundary joints your design needs.
10 × 100 + 9 × 10 = 1 090 mm
Work backwards from the available width
With 1090 mm, no end joints and ten 100 mm bricks, the remaining space is 1090 − 1000 = 90 mm. Divide this by nine internal joints to get 10 mm per joint. Try another whole brick count if the result does not suit the layout; do not silently absorb the remainder into thin cuts.
Why jack arches need a different division
Jack arch lines radiate from a setting-out centre. Each cut piece can therefore have different widths at its bottom and top. A curved arch also needs measurement along the chosen arc. Do not simply divide the clear opening by a brick dimension. Use the calculated piece sizes, joints and bond for that shape.