Why does the calculator add 54 % to the finished volume?

Dry sand and aggregate contain air voids; the extra 54 % ensures you batch enough loose material to achieve the required compacted volume.

Is the water-to-cement ratio entered by weight or volume?

Enter it by weight-e.g. 0.55 means 0.55 kg of water for every kilogram of cement.

Can I use non-integer mix ratios such as 1 : 1.5 : 2.8?

Yes. The parts for cement, sand and gravel can be any positive decimal; the calculator scales volumes automatically.

What densities are assumed for the materials?

The tool uses 1440 kg/m^3 for cement, 1536.491 kg/m^3 for sand and 1589.98066 kg/m^3 for gravel, which align with common bulk-density references.

Spike's Calculators

Metric Concrete Slab Mix Calculator

Self Mix Concrete Calculator. AI generated image

The Metric Concrete Slab Mix Calculator calculates the precise amounts of cement, sand, gravel, and water needed to pour a flat slab, such as a patio, path, or shed floor, when measurements are supplied in metres, millimetres, and kilograms.

Background

Volume-based mix ratios (1:2:3 and similar) became standard in early 20th-century building guides, enabling tradespeople to batch concrete on-site using shovels or buckets. The modern practice still uses those parts but applies a 54 % bulking factor to account for voids between dry particles before compaction. This calculator automates that long-hand method and adds weight conversions for accuracy.

Self-Mix Slab Metric Calculator

Mixing Ratio

Cement  #
Sand      #
Gravel    #

Cement/Water Ratio
#

Length of Slab
m

Width of Slab
m

Depth of Slab
mm

Results:

Volume of Concrete Needed
m³

Dry Volume of the Ingredients
m³

Volume and Weight of Cement
m³ kg

Volume and Weight of Sand
m³ kg

Volume and Weight of Gravel
m³ kg

Water
m³
kg
litres

Total Weight
kg

Calculator

enter the desired mixing ratios for cement, sand, and gravel in the mix ratio fields
the cement-to-water ratio for the desired consistency of the concrete
the length of the slab in metres
the width of the slab in metres
the depth of the slab in millimetres

Results

the volume of concrete required, in cubic metres (m³)
volume of the dry materials in cubic metres
volume and weight of cement in cubic metres and kilograms
volume and weight of sand in cubic metres and kilograms
volume and weight of gravel in cubic metres and kilograms
the water volume in cubic meters, weight in kilograms, and water in litres
the total weight of all materials in kilograms

Concrete Slab Mix - Calculation Formulas

$$V = L \times W \times \left(\frac{D}{1000}\right)$$ $$V_d = V \times 1.54$$ $$P = c + s + g$$ $$V_c = \frac{c}{P}\,V_d,\;\;V_s = \frac{s}{P}\,V_d,\;\;V_g = \frac{g}{P}\,V_d$$ $$M_c = V_c \times 1440,\quad M_s = V_s \times 1536.491,\quad M_g = V_g \times 1589.98066$$ $$M_w = \left(\frac{w}{c}\right) M_c,\qquad V_w = \frac{M_w}{1000}$$ $$M_{\text{total}} = M_c + M_s + M_g + M_w$$

Worked example

Slab 4 m x 3 m x 100 mm, mix 1 : 2 : 3, w/c = 0.55

Step	Equation	Result
Finished volume	$V = 4 \times 3 \times 0.1$	1.200 m³
Dry volume	$V_d = 1.200 \times 1.54$	1.848 m³
Total parts	$P = 1+2+3$	6
Cement vol.	$V_c = \tfrac16 \times 1.848$	0.308 m³
Sand vol.	$V_s = \tfrac26 \times 1.848$	0.616 m³
Gravel vol.	$V_g = \tfrac36 \times 1.848$	0.924 m³
Cement kg	$M_c = 0.308 \times 1440$	444 kg
Sand kg	$M_s = 0.616 \times 1536.491$	946 kg
Gravel kg	$M_g = 0.924 \times 1589.98066$	1469 kg
Water kg	$M_w = 0.55 \times 444$	244 kg
Water L	$M_w$	244 L
Total weight	$M_{\text{total}}$	3103 kg

This tool is designed for users who need to mix concrete on-site for slab construction. It ensures accurate material quantities based on slab dimensions.

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