Ordering too little shuttering stops a pour halfway. Ordering too much means paying rent on material that sits in a corner. This guide shows a quick way to estimate what a slab needs before you ask for a quote.
Step 1: Measure the slab area
Multiply the slab length by its width. For an L-shaped or irregular slab, split it into rectangles, work out each one, and add them up. Leave out openings such as stair cut-outs and ducts.
Example: a slab of 40 ft × 25 ft = 1,000 sq ft (about 92.9 m²).
Step 2: Work out the number of plates
Divide the slab area by the area of one plate:
- 1200 × 600 mm plate = 0.72 m² (about 7.75 sq ft)
- 900 × 600 mm plate = 0.54 m² (about 5.81 sq ft)
Then add around 5–10% extra for edges, cuts and damaged plates.
Example: 1,000 sq ft ÷ 7.75 sq ft ≈ 129 plates. Adding 8% gives roughly 140 plates of 1200 × 600 mm.
Step 3: Estimate the props
Props are usually set out in a grid. Count how many fit along each side at your chosen spacing, then multiply:
Props ≈ (length ÷ spacing + 1) × (width ÷ spacing + 1)
Example: at about 1.0 m (3.3 ft) spacing, a 12.2 m × 7.6 m slab needs about (12 + 1) × (8 + 1) = 117 props.
The right spacing depends on slab thickness, runners and prop capacity. Read our prop spacing guide before you finalise it.
Step 4: Don't forget beams, edges and accessories
- Beam sides and bottoms need their own plates. Measure them separately.
- Slab edges need side shuttering at the slab thickness.
- Runners or acro spans carry the plates between props.
- Clamps, wedge pins and U-jacks complete the formwork.
Step 5: Plan for cycles
If you're casting several floors one after another, you don't always need a full set for every floor. Depending on your striking times, one set of plates may move up while props stay below for back-propping. Plan the sequence first, then the quantity.
This is a planning estimate, not a structural design. Prop spacing and formwork for your slab should be confirmed by a qualified engineer.