CalcConcrete Calculator
Last updated: 2026-09-14
How to Use This Calculator
This concrete calculator helps you determine the volume of concrete needed for a foundation slab and the required amounts of cement, sand, and gravel. Simply enter the slab dimensions—length, width, and thickness—and choose whether your measurements are in meters or centimeters. The calculator then computes the volume in cubic meters and estimates the mass of each ingredient based on a standard 1:2:3 mix by mass (cement:sand:gravel). It also tells you how many 25 kg or 50 kg bags of cement you will need.
Formula and Methodology
The volume of a rectangular slab is calculated as: Volume = Length × Width × Thickness. All dimensions must be in the same unit; if you enter centimeters, the calculator converts them to meters. The total mass of concrete is estimated using a typical density of 2400 kg/m³. The mix ratio of 1:2:3 by mass means that for every 1 part cement, you need 2 parts sand and 3 parts gravel. Therefore, cement makes up 1/6 of the total mass, sand 2/6, and gravel 3/6. The amount of cement is then divided by 25 or 50 to determine the number of bags.
Practical Examples
Suppose you are building a shed foundation that is 5 meters long, 4 meters wide, and 15 centimeters thick. Convert thickness to meters: 0.15 m. Volume = 5 × 4 × 0.15 = 3 m³. Total mass = 3 × 2400 = 7200 kg. Cement = 7200 × (1/6) = 1200 kg. Sand = 7200 × (2/6) = 2400 kg. Gravel = 7200 × (3/6) = 3600 kg. For cement bags: 1200 / 25 = 48 bags (25 kg) or 24 bags (50 kg).
Tips and Best Practices
Always add 5–10% extra to account for spillage, uneven subgrade, and waste. If you are using a different mix ratio, adjust the quantities accordingly. For large projects, consider ordering ready-mix concrete to save time and ensure consistency. Store cement bags in a dry place and use them within a few months. When mixing by hand, measure ingredients by weight for accuracy, or use a known volume ratio (e.g., 1 part cement, 2 parts sand, 3 parts gravel by volume) but note that volume proportions differ from mass proportions. Finally, ensure proper curing by keeping the concrete moist for at least 7 days.
FAQ
How do I calculate concrete volume for a slab?
Multiply length × width × thickness (all in meters). For example, a slab 5 m long, 4 m wide, and 0.15 m thick has a volume of 3 m³.
How many 25 kg bags of cement do I need for 1 m³ of concrete?
With a 1:2:3 mix by mass, you need about 400 kg of cement per m³, which equals 16 bags of 25 kg.
What is the ratio for concrete?
A common general-purpose mix is 1 part cement, 2 parts sand, and 3 parts gravel by mass. This calculator uses that ratio.
Should I order ready-mix or mix on site?
For volumes over 1 m³, ready-mix is often more convenient. For small jobs, bagged cement and aggregates may be cost-effective.