Calculating Concrete Shapes and Screed Bag Yields

Collect geometry and product information separately

Write down the finished concrete dimensions or the screed thickness between the existing substrate and target surface. Convert lengths to metres before multiplying: 150 mm = 0.15 m, and 1 m³ = 1,000 L. Identify each region on a sketch so no volume is counted twice.

For the product, record its full name, market, bag size, technical-sheet revision, mixed yield or depth-specific coverage, and intended application. Bag weight alone does not establish mixed volume. Water dosage is also not yield: do not add extra water to increase coverage.

Solid steps: measure three bands, not three full-height boxes

For a solid flight on a level base, record the concrete width, equal rise, equal going and number of rectangular bands shown. In this convention three bands have lengths of three, two and one goings. Use the Concrete Steps Calculator (C10) only when the measured shape matches that model; it does not select safe stair dimensions.

Worked example: three bands, 0.30 m going, 0.15 m rise and 1.00 m width, with no landing or void. The dimensions are illustrative quantities, not a stair-design recommendation.

1. Bottom band: (3 × 0.30 m) × 0.15 m × 1.00 m = 0.135 m³.
2. Middle band: (2 × 0.30 m) × 0.15 m × 1.00 m = 0.090 m³.
3. Top band: 0.30 m × 0.15 m × 1.00 m = 0.045 m³.
4. Add non-overlapping volumes: 0.135 + 0.090 + 0.045 = 0.270 m³, or 270 L.
5. Apply the example’s entered 10% purchasing allowance once: 270 L × 1.10 = 297 L.
6. With an illustrative concrete premix yield of 20 L/bag: 297 L ÷ 20 L/bag = 14.85 bags. Round upward to 15 bags, supplying 300 L.

The compact formula is width × rise × going × n(n + 1)/2; for n = 3 the multiplier is 6. Do not add three boxes at heights 0.15, 0.30 and 0.45 m on top of these bands: that would count the same solid twice. Hollow, void-backed, suspended, curved or sloping-base steps need a different quantity model.

Landings: add only a separately measured solid

Measure landing length, width and actual concrete thickness independently. Confirm that it neither overlaps the step solid nor is already included in a band. A separate 1.00 m × 1.20 m × 0.10 m landing is 0.120 m³. Added to the flight above, net concrete becomes 0.270 + 0.120 = 0.390 m³.

If it uses the same specified concrete, aggregate those volumes before applying the allowance and bag rounding: 390 L × 1.10 ÷ 20 L/bag = 21.45, rounded up to 22 bags. This landing is excluded from the 15-bag example. Keep separate product orders when the landing and steps require different mixes. The Concrete Bags Calculator converts a measured total using editable yields; match its custom yield and allowance to your product rather than treating its defaults as a specification.

Floor screed: measure thickness across the whole area

Screed forms a specified floor layer; it is not a substitute for structural concrete. Record the target level and the thickness to the substrate over measured zones. A uniform zone uses area × thickness. For an uneven floor use an area-weighted average: total volume divided by total area, retaining the separate zone measurements.

Endpoint averaging is valid for an explicit rectangular zone with a linear thickness change across its length and consistent thickness across its width. It is not valid for arbitrary high and low spot readings. A 5 m × 4 m rectangle tapering from 30 mm to 70 mm has average thickness (30 + 70)/2 = 50 mm; its volume is 20 m² × 0.050 m = 1.000 m³. Irregular depressions require separately measured zones rather than that shortcut.

Worked premixed screed example: 20 m², 1 m³, 84 bags

A separate 20 m² floor at a uniform 50 mm depth has the same volume as the rectangular taper above. Use the Screed Quantity Calculator (C12) with the appropriate uniform or taper geometry. Assume an illustrative premixed screed yield of 12.5 L/bag and an entered 5% allowance.

1. Convert depth: 50 mm ÷ 1,000 = 0.050 m.
2. Net volume: 20 m² × 0.050 m = 1.000 m³ = 1,000 L.
3. Allowance-adjusted need: 1,000 L × 1.05 = 1,050 L.
4. Bag need: 1,050 L ÷ 12.5 L/bag = 84 bags. Already whole, so the purchased count remains 84.

At 0% allowance the same screed example needs 80 bags. Both yields and allowances in these examples are editable assumptions, not verified QUIKRETE or ARDEX product presets. The concrete example uses 20 L/bag for a different purpose; the screed example uses 12.5 L/bag. Neither yield transfers to the other product.

Binder versus premix: what is actually in the bag?

A premixed screed includes its specified aggregate. A binder-only screed requires separately supplied aggregate with the manufacturer’s grading and mix proportions. Count binder and aggregate as distinct materials; a litre-per-premix-bag value cannot price or order a binder-only bag.

S10: ARDEX Americas’ A 38 lists screed sand added to its binder, while A 38 MIX is premixed with sand. Their bag contents and coverage bases differ. Obtain the matched technical sheet and complete aggregate specification for a binder system; C12’s premixed bag mode does not calculate that system.

If the selected premix instead specifies dry consumption in kg/m²/mm, use that exclusive basis: sum each zone’s area × thickness in mm × stated consumption, apply the allowance once, then divide by dry bag mass and round upward. Do not derive dry mass from fresh density or apply wet-yield and dry-consumption conversions together.

Self-leveling is another product selection

Use the Self-Leveling Compound Calculator when the specified product is a self-leveling floor preparation material. Enter its bag coverage at the stated reference depth and your measured application depth. Check permitted thickness, substrate, primer and application requirements against that product’s documentation. A floor screed, a self-leveler and structural concrete are not interchangeable merely because all occupy a measurable volume.