About

Ordering one bag too few halts a pour mid-slab. Ordering ten too many wastes $80+ and storage space. The core problem is converting between weight printed on a bag label and the volume that material actually occupies once spread or poured. Every material has a different bulk density: Portland cement packs at roughly 1506 kg/m³, while dry garden mulch sits near 320 kg/m³. This tool uses published bulk-density constants to bridge the weight - volume gap, then divides by your chosen bag size and rounds up, because suppliers do not sell partial bags. It also computes coverage area at a given depth for spread materials like gravel, topsoil, and decorative stone.

Inputs assume loosely packed (not compacted) densities. For tamped or wet conditions, actual density can rise 10 - 30%. Add a 5 - 10% waste factor for spills, uneven surfaces, and bag residue. Pro tip: cement absorbs moisture in storage and can gain 5% mass within weeks, so buy close to your pour date.

Formulas

The number of bags required depends on whether you know the total volume needed or the area to cover at a specific depth.

V_total = L × W × D

For coverage mode, D is the spread depth. For fill mode, D becomes the height H of the space to fill.

V_bag = W_bagρ

This converts a bag labeled by weight into its actual volume using the material bulk density ρ. If the bag is already labeled in liters, this step is skipped.

N = ⌈ V_total × (1 + w)V_bag ⌉

The ceiling function ensures you always round up to a whole bag. The waste factor w (typically 0.05 to 0.10) accounts for spillage and uneven application.

C = N × P

Where L = length, W = width, D = depth/height, W_bag = bag weight, ρ = bulk density kg/m³, V_bag = volume per bag, w = waste factor, N = number of bags, P = price per bag, C = total cost.

Reference Data

Material	Bulk Density	Common Bag Sizes	Typical Use	Notes
Portland Cement	1506 kg/m³	25, 50 kg	Concrete, mortar	Store dry; absorbs moisture
Ready-Mix Concrete	2200 kg/m³	20, 25, 30 kg	Footings, posts, slabs	Add water only
Sand (dry)	1600 kg/m³	25, 50 kg	Bedding, mortar mix	Wet sand ≈ 1900 kg/m³
Gravel (loose)	1550 kg/m³	25, 50 kg	Drainage, paths	Size affects packing
Pea Gravel	1680 kg/m³	20, 25 kg	Landscaping, drainage	Rounded, low compaction
Crushed Stone	1600 kg/m³	25, 50 kg	Sub-base, fill	Angular; compacts well
Topsoil	1100 kg/m³	20, 25, 40 L	Garden beds, lawns	Organic content varies
Potting Mix	480 kg/m³	25, 50 L	Containers, pots	Very light; peat-based
Compost	550 kg/m³	25, 40 L	Soil amendment	Density varies with moisture
Mulch (wood chip)	320 kg/m³	50, 80 L	Weed suppression	Decomposes over 1 - 2 years
Bark Chips	340 kg/m³	50, 70 L	Decorative, paths	Floats; avoid slopes
Perlite	100 kg/m³	100 L	Soil aeration	Extremely light
Vermiculite	120 kg/m³	100 L	Moisture retention	Absorbs 3 - 4× its weight
Play Sand	1500 kg/m³	20, 25 kg	Sandboxes, leveling	Washed, rounded grains
Mortar Mix	1650 kg/m³	20, 25 kg	Bricklaying, pointing	Pre-blended cement + sand
Decorative Stone	1500 kg/m³	20, 25 kg	Landscaping	Density varies by stone type
Fertilizer (granular)	900 kg/m³	5, 10, 25 kg	Lawn, garden feeding	Follow application rate
Rock Salt	1200 kg/m³	10, 25 kg	De-icing	Corrosive to metal
Limestone (crushed)	1540 kg/m³	25, 50 kg	Driveways, pH amendment	Compacts firmly
Clay (dry)	1600 kg/m³	25 kg	Pottery, fill	Expands when wet

Frequently Asked Questions

Wet material is denser than dry. Wet sand can reach 1900 kg/m³ versus 1600 kg/m³ dry. If you measure volume on-site from a wet pile, fewer bags (sold by dry weight) will fill it. Conversely, if you calculate based on dry density but the bags were stored in humid conditions, each bag may weigh more than labeled but occupy roughly the same volume. For critical pours, weigh a single bag to verify before scaling.

Heavy materials (cement, sand, gravel) are sold by weight in kg because their density is consistent. Light materials (mulch, potting mix, perlite) are sold by volume in liters because weight would be misleadingly small. This calculator handles both. Select the correct bag unit type - the conversion uses bulk density ρ to bridge the two.

Partial bags do not exist at retail. If you need 7.1 bags, you buy 8. The waste factor (default 5%) covers material left in bags, uneven substrate, minor measurement errors, and compaction losses. For irregular shapes or rocky ground, increase waste to 10-15%. For precision pours with formwork, 5% is sufficient.

Enter the equivalent rectangular area. For a circle of radius r, the area equals π × r². Compute this value and enter it as Length × Width where one dimension is 1. For example, a circle with radius 3 m has area ≈ 28.27 m². Enter length 28.27 and width 1. The depth field remains your desired thickness.

Yes. Crushed stone compacts roughly 15-25% from loose state. If you need 100 mm compacted depth, order material for 120-125 mm loose depth. This calculator uses loose bulk density by default. Increase the depth input or the waste factor to compensate for compaction requirements.

Select "Custom" and enter the bulk density manually. Check the material's Safety Data Sheet (SDS) or supplier spec sheet - bulk density is usually listed in Section 9 (Physical Properties). If unavailable, fill a known container (e.g., a 10 L bucket), level it off, and weigh it. Density = mass / volume.