How to estimate a villa

A villa estimate starts with drawings, not with a price. This article sets out a repeatable order for taking dimensions off a house plan and turning them into quantities: blockwork and mortar, plaster, reinforcing steel, floor tiles, paint, roofing sheets and concrete. By the end a reader can work through the construction material calculator and the concrete calculator and know which inputs still need a decision from an engineer or a specification.

What order should the quantities be worked out in?

Work through the materials in the order the work happens on site: walls and their joints, then plaster, then steel, then floors and paint, then roof, with concrete quantities for foundations, columns, beams and slabs taken alongside the elements they belong to. Each material uses an area or a volume you have already measured.

1. Gather the floor plans, elevations and sections. Note wall lengths and heights, opening sizes, floor areas, roof slopes and element thicknesses.
2. Start with blockwork. The wall area is length times height, less the doors and windows. A 10 m wall 3 m high with 3 m2 of openings is 27 m2.
3. Take the mix and the waste percentage for each material from the project specification, or set your own and record it.
4. Move to plaster over the same wall areas, then to reinforcing steel from the bar bending schedule, then to floor tiles, paint, roofing sheets and concrete.
5. Keep the drawing revision and the date with every set of figures. Re-run the sums when the drawings change.

The calculator returns quantities only. It adds no labour, transport or cost, so anyone building a budget should carry the quantities into the headings a budget usually has, the list set out on the construction budget analyzer page as a prompt to check rather than a standard: preliminaries and site set-up; substructure and foundations; frame and structure; walls; roof; doors and windows; finishes; electrical services; plumbing and drainage; external works; professional fees; contingency. For the wider commercial picture, the business and costs section of this site collects the related reading.

How do you count blocks and the mortar in the joints?

Blocks to buy equal the wall area times the blocks per square metre times one plus the waste percentage, rounded up, and the mortar is the volume the joints hold. The area is length times height less openings, and one block with its joints covers (block length + joint) times (block height + joint).

Worked example from the tool's own method:

Lintels, stiffening columns and bond beams are concrete work, not blockwork, so they come out of the wall area and go into the concrete take-off. Block sizes sold locally vary, so confirm the length, height and joint the mason will use before ordering. A construction material calculator run with those three figures gives blocks and mortar for any wall on the plan.

How is plaster, tiles and paint estimated?

Plaster uses wet volume times a dry volume factor, tiles are counted from the floor area and the tile size, and paint is litres equal to area times coats divided by coverage per coat, with waste added to each. All three start from an area you have already measured.

Plaster example: 50 m2 at 15 mm with 10% waste is 0.825 m3 of plaster. At 1 : 4 with a factor of 1.3 that is 1.073 m3 dry, 308.9 kg of cement (7 bags of 50 kg) and 0.858 m3 of sand.

Tiles example: a 20 m2 floor in 600 x 600 mm tiles with 3 mm joints gives a cover of 0.3636 m2 per tile, 55 tiles laid, or 61 with 10% waste, which is 16 boxes of four. Buy all the tiles from one batch, because shades vary between production runs.

Paint example: 100 m2 in two coats at 10 m2 a litre with 5% waste is 21 litres. Coverage per coat and the number of coats come from the product data or the specification.

Steel sits outside these three because it comes from the bar bending schedule rather than from an area. Mass per metre is steel density times pi over four times diameter squared. At 7,850 kg/m3 that is diameter squared divided by 162, with diameter in mm and mass in kg per metre. Total length is the number of bars times the cut length times one plus waste. Twenty 12 mm bars cut at 6 m are 120 m, or 126 m with 5% waste. A 12 mm bar weighs 0.888 kg a metre, so 111.9 kg. Two 6 m bars come out of each 12 m stock bar, which is 11 stock bars. Laps, bends and hooks belong in the cut length, and the grade and the diameters come from the drawings.

How do you work out concrete for the slab, columns and footings?

Concrete volume comes from the element's own dimensions, plus waste, multiplied by a dry volume factor and then split by the mix, with cement converted to whole bags and water taken from the water-cement ratio. Slabs, beams, columns and pad footings are each measured as their own shape.

Worked example from the concrete calculator: a ground-floor slab 5 m long, 4 m wide and 150 mm thick at 1 : 2 : 4 with 5% waste takes 3.15 m3 of concrete, which is 20 bags of 50 kg of cement, 1.386 m3 of sand, 2.772 m3 of aggregate and 499 litres of water.

1. Measure each element: length, width, thickness for slabs and footings; cross-section and height for columns and beams.
2. Add the waste percentage for the pour.
3. Apply the dry volume factor the tool states as an assumption and split the dry volume by the mix parts.
4. Convert cement to whole 50 kg bags and calculate water from the water-cement ratio.
5. Repeat for each element and sum by pour, so the order matches the site programme.

What does the roofing sheet count include, and what does it leave out?

Sheets are counted in rows up each slope and sheets along each row, so the count depends on the slope length, the sheet length, the effective cover width and the end lap. A roof 10 m long with two 4 m slopes, in 3 m sheets covering 0.8 m each with 150 mm end laps, gives two rows per slope and 13 sheets per row, 52 sheets, or 55 with 5% waste.

1. Divide the slope length by the sheet length to get the rows per slope, allowing the end lap.
2. Divide the length along the ridge by the effective cover width and round up for the sheets in a row.
3. Multiply rows by sheets per row by the number of slopes.
4. Add the waste percentage and round up.

Ridge caps, flashings, fixings and purlins are extra and are not in the sheet count. Take their number from the roof layout and the supplier's component list.

Which assumptions do you set, and what still needs an engineer?

The waste allowance, the dry volume factor, the bulk density and the product sizes are assumptions you can change, and every quantity comes from your own drawings and measurements. The calculator states each figure as an estimate for planning, not a design.

Keep a short assumption sheet with the estimate: drawing revision, block size and joint, mortar and plaster mixes, dry volume factor, bulk density, steel grade, tile size and joint, paint coverage per coat, number of coats, sheet length and effective cover width, end lap, waste percentage for each material, and the concrete mix and water-cement ratio. When you ask for a second opinion, hand over that sheet with the drawings. A structural engineer confirms the mix, the bar schedule and the structural sizes, and the specification confirms the finishes and the products. The education page carries the related background reading for readers new to quantity take-off.

What to do next

Open the construction material calculator with the wall lengths and heights, the opening sizes, the floor areas, the roof slope and length, the block and tile sizes, the joints, the mixes, the coverage per coat and your waste percentages. Take the slab, column, beam and footing dimensions to the concrete calculator with the mix and the water-cement ratio. Keep the drawings and the assumption sheet beside the results.