How many bricks (voussoirs) in an arch?

The number of voussoirs = Arc Length / (Brick Length + Joint Width). For a segmental arch over a 1.8m opening with 300mm rise: radius ≈ 1.5m, arc length ≈ 2.0m. With 215mm bricks and 3mm joints: approximately 9 voussoirs. Always use an odd number of voussoirs so there is a key brick at the crown. Arches should always have an odd voussoir count.

What is a skewback angle?

The skewback is the angled surface at each end of the arch (the springing point) where the arch meets the vertical wall. The skewback angle depends on the arch geometry — for a semicircular arch it's 90° (horizontal springing). For a segmental arch, it's less than 90°. The bricks at the springing point must be cut to this angle for proper bearing.

What is a temporary centre for an arch?

A temporary centre (or former) is a shaped timber support built to the exact profile of the arch underside. Bricks are laid on top of the centre, which maintains the arch shape during construction. The centre must stay in place until the mortar has cured (typically 7-14 days). It is then carefully removed ('struck'), allowing the arch to become self-supporting.

What mortar for brick arches?

Use a strong mortar mix for arches — typically 1:3 cement:sand (M6) or 1:1:6 cement:lime:sand (M4). The joints between voussoirs should be very thin (2-5mm) for gauged arches using rubbed bricks, or up to 10mm for rough arches. Lime mortar is traditional and allows slight movement, but cement mortar provides faster strength gain.

What is the difference between segmental and semicircular arches?

A semicircular arch has a rise equal to half the span (forming a perfect half-circle). A segmental arch has a rise less than half the span (forming a shallow arc). Segmental arches are more common in domestic buildings as they require less headroom. Flat gauged arches have a very slight camber (2% of span) and require specially shaped bricks.

TradeCalculator

Brick Arch Calculator — Geometry, Voussoirs & Skewback

Calculate brick arch geometry, voussoir count, skewback angle and temporary centre for arch construction.

Arch Span

Clear opening width

Arch Type

Rise

0 = default (span/4)

Arch Depth (Ring Depth)

Number of brick courses deep

Brick Price

How We Calculate This

This calculator determines brick arch geometry, voussoir count, skewback angle and temporary centre dimensions.

Geometry

Semicircular: Radius = Span / 2, Rise = Span / 2
Segmental: Radius = (Span²/4 + Rise²) / (2 × Rise)
Arc Length = 2 × Radius × arcsin(Span / (2 × Radius))

Voussoirs

Voussoirs = Arc Length / (Brick Length + Joint Width)
Use an odd number for a central key brick. Skewback angle = half-angle of the arc in degrees.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.