Concrete worked example: 6 m x 4 m slab, 100 mm deep

BuildTrue concrete pack. Estimates only. This example uses the same formulas as the concrete calculator on the pack page and the standalone BuildTrue concrete estimator. Verified by hand, step by step, below.

Inputs

Input	Value
Slab length	6 m
Slab width	4 m
Slab depth	100 mm (0.100 m)
Waste allowance	10 percent
Bag yield, stated assumption	One 30 kg pre-mix bag yields about 0.0135 cubic metres
Conversion, stated	1 cubic metre = 1.30795 cubic yards

Step by step arithmetic

1. Volume before waste:

6 x 4 x 0.1 = 2.40 cubic metres.

1. Volume with waste:

2.40 x (1 + 10 / 100) = 2.40 x 1.10 = 2.64 cubic metres.

1. Volume in cubic yards:

2.64 x 1.30795 = 3.453 cubic yards, shown as 3.45 cubic yards. Before waste: 2.40 x 1.30795 = 3.139 cubic yards, shown as 3.14.

1. Bags:

2.64 / 0.0135 = 195.5556 bags. Round up with the epsilon safe ceiling: 196 bags. Sanity check: 195 bags x 0.0135 = 2.6325 cubic metres, which is short of 2.64. 196 bags x 0.0135 = 2.646 cubic metres, which covers it. So 196 is the smallest whole bag count that works.

Purchasable counts

Item	Raw result	Buy
30 kg pre-mix bags	195.56 bags	196 bags
Ready-mix alternative	2.64 cubic metres with waste	Order to the supplier increment above 2.64 cubic metres. If the supplier sells in 0.5 cubic metre steps, that is 3.0 cubic metres. Confirm the increment, the minimum load, and any short load fee before pour day

Sample material list for this job

Item	Specification	Quantity
Pre-mix concrete	30 kg bags, 0.0135 cubic metres yield each	196 bags (or ready-mix as above, not both)
Granular base	Compacted depth as specified	Volume from base area and compacted depth, ordered separately
Reinforcement	Rebar or mesh as specified	Per layout, with laps. Not counted by the slab calculator
Form lumber and stakes	To suit 100 mm depth and 20 m of perimeter (6 + 4 + 6 + 4)	Perimeter run plus bracing
Curing and joint consumables	As specified	Per label coverage

Perimeter check used in the list: 6 + 4 + 6 + 4 = 20 m of form run.

Notes and limits

• The calculator covers a simple rectangle at a uniform depth. Thickened edges, pads, steps, and sloped bases add volume and must be added by hand.
• Bag mixing 196 bags is a large hand pour. The arithmetic is the same, but most crews would price the ready-mix alternative for a slab this size and choose on labour, access, and timing.
• Estimates only. This is not engineering or code compliance advice. Verify quantities, mix, reinforcement, and local requirements with a qualified professional before ordering or pouring.