How the stucco calculator works
Stucco in North America and render in Europe are the same idea: a cement or lime mortar troweled onto a wall in layers. The base coats (a scratch coat, then a brown or levelling coat) are thick and use most of the material; the finish coat is thin, finer and often coloured, and it is sold as a different product. That is why the calculator counts bags for each layer separately. It works out the net wall, multiplies by each thickness and by a dry-mix consumption of 1.7 kg per m² for every millimetre, and adds 15% for mortar dropped off the hawk and left in the mixer.
Dry mix (kg) = (length × height − openings) × thickness (mm) × 1.7 × 1.15In US units the wall is converted to m² and the thickness to millimetres before the same sum, and the result is shown in pounds. 1.7 kg/m² per mm is equivalent to about 106 lb of dry mix per cubic foot of finished render.
Worked example
A single-storey extension has 30 m of wall 2.6 m high, or 78 m². Windows and a patio door take up 12 m², leaving 66 m². A 12 mm base on blockwork uses 66 × 12 × 1.7 = 1,346 kg, and 1,548 kg with the 15%, so 62 bags of 25 kg. A 3 mm finish coat uses 66 × 3 × 1.7 × 1.15 = 387 kg: 16 bags. Altogether that is about 1.9 tonnes of dry mix, a full pallet, so arrange delivery near the mixing spot.
Coat systems and thickness
| System | Typical thickness | Where |
|---|---|---|
| Three-coat stucco over lath | 3/8 + 3/8 + 1/8 in (about 22 mm) | Timber or steel framing with wire lath |
| Two-coat stucco on masonry | 1/2–5/8 in total (13–16 mm) | Concrete block, brick |
| One-coat stucco | 3/8–1/2 in (10–13 mm) plus finish | Over lath and foam board, per product listing |
| Traditional sand-cement render | 10–12 mm scratch + 6–8 mm top | UK and European blockwork |
| Through-coloured one-coat render | 12–15 mm in two passes | Block and brick facades |
What one bag covers
| Bag | Thickness | Covers about |
|---|---|---|
| 25 kg | 6 mm | 2.5 m² |
| 25 kg | 10 mm | 1.5 m² |
| 25 kg | 15 mm | 1.0 m² |
| 80 lb | 1/8 in | 72 ft² |
| 80 lb | 3/8 in | 24 ft² |
| 80 lb | 7/8 in | 10 ft² |
Avoiding cracks and failures
- Keep it damp. Cement stucco cures, it does not dry; mist the base coat for at least two days in warm or windy weather.
- Do not apply in frost or hot sun. Below about 5 °C (40 °F) or on a baking wall the mortar weakens and crazes.
- Fit beads and joints. Corner beads, stop beads and a weep screed at the base are needed, and large walls need control joints. Over framing, the lath, fixing and water-resistive barrier must follow the local building code.
Frequently asked questions
How many bags of stucco do I need per square foot?
An 80 lb bag covers about 24 ft² at 3/8 in, so a two-coat 1/2 in base needs roughly one bag per 18 ft², plus waste.
How much render do I need per square metre?
About 1.7 kg per m² for every millimetre, so a 15 mm render takes roughly 25 kg per m² before waste.
How thick should stucco be?
Traditional three-coat stucco over lath is about 7/8 in (22 mm) thick; two coats on block or brick are about 1/2–5/8 in.
How many square metres does a 25 kg bag of render cover?
About 1.5 m² at 10 mm and 1 m² at 15 mm, less on uneven blockwork that needs dubbing out.
Can stucco be applied directly to concrete block?
Yes, on clean, sound block with good suction a bonding or scratch coat can go straight on; smooth or painted surfaces need a bonding agent or lath.
Last reviewed: 2026-09-27 · How we calculate