Cement Calculator
Calculate bags of cement needed for concrete and mortar projects.
Cement Calculator
Use the measurement fields above to calculate your results instantly.
What This Calculator Does
The Cement Calculator estimates how many bags of cement or pre-mixed concrete you need for slabs, footings, post holes, and mortar projects. Enter your project volume in cubic feet or cubic yards, and the calculator returns the number of 40 lb, 50 lb, 60 lb, 80 lb, and 94 lb bags required including your chosen waste factor.
Cement is the binding ingredient in concrete and mortar. Concrete is a mixture of cement, sand, gravel, and water. Mortar is cement mixed with sand and lime for masonry work. This calculator handles both pre-mixed concrete bags (which include cement, sand, and aggregate) and bulk Portland cement estimates for custom mixing. Understanding the difference between cement, concrete, and mortar is essential for ordering the right material. Cement is always an ingredient - never a standalone finished material - while concrete and mortar are the finished mixtures used in construction.
How to Use
- Measure the length, width, and depth of your project in feet and inches. For slabs, multiply all three dimensions to get cubic feet. For post holes, use the hole diameter and depth.
- Convert your total volume to cubic feet (1 cubic yard = 27 cubic feet). Enter the total volume into the calculator.
- Select your bag type: 40 lb, 60 lb, 80 lb pre-mixed concrete bags, or 94 lb bulk Portland cement bags.
- Select a waste factor - 5% for simple forms, 10% for irregular shapes, 15% for post holes and complex projects.
- Click "Calculate" and round up to the nearest full bag. Bags must be purchased whole - suppliers cannot split them.
How to Calculate Cement
Calculating how many bags of cement or concrete you need requires three steps: compute the project volume, determine the bag yield, and apply the waste factor.
Step 1: Calculate Project Volume in Cubic Feet
For a rectangular slab: Volume (cu ft) = Length (ft) x Width (ft) x Depth (ft). Convert inches to feet by dividing by 12. A 10 x 10 ft slab at 4 inches thick: 10 x 10 x (4/12) = 33.3 cu ft. For post holes: Volume = π x Radius x Radius x Depth, where radius is half the hole diameter. For round footings, use the same cylinder formula.
Step 2: Determine Bag Yield
Pre-mixed concrete bag yields vary by weight. A standard 80 lb bag yields approximately 0.6 cu ft of concrete. A 60 lb bag yields 0.45 cu ft. A 40 lb bag yields 0.3 cu ft. For bulk Portland cement (94 lb bag), the cement itself yields 1 cu ft, but when mixed with sand and gravel it produces about 4.5 cu ft of finished concrete at a standard 1:2:3 mix ratio. To find the number of bags: Bags = Total Volume / Bag Yield.
Step 3: Apply Waste Factor
Multiply your bag count by (1 + waste percentage). For a typical slab with 10% waste: 10 bags x 1.10 = 11 bags. Always round up to the nearest full bag. Waste accounts for spillage, uneven subgrades, over-dig, and leftover material in the mixer.
Converting Between Cubic Feet and Cubic Yards
Ready-mix concrete is ordered by the cubic yard. To convert: Cubic Yards = Cubic Feet / 27. For a 10 x 10 slab at 4 inches: 33.3 cu ft / 27 = 1.23 cu yd. Most ready-mix trucks carry 8-10 cu yd. For projects under 2 cu yd, bagged concrete is usually more practical than scheduling a delivery.
Practical Measurement Tips
- Measure the actual form dimensions. A 2x4 form board is actually 3.5 inches wide, not 4 inches. When calculating slab depth, use the actual board width if you are using dimensional lumber for forms. An advertised 4-inch slab using 2x4 forms gives you exactly 3.5 inches - a 12.5% reduction in volume.
- Compact the subgrade. Loose soil compresses under concrete weight, increasing your slab thickness by 1/4 to 1/2 inch on average. Compact the soil with a hand tamper before forming to reduce concrete consumption and prevent cracking.
- Mix in batches. For projects requiring 10 or more bags, mix in manageable batches of 2-3 bags at a time. Concrete starts setting in 30-60 minutes depending on temperature. Mixing too many bags at once risks having the first batch harden before you place the last batch.
- Use the right bag type. For structural slabs and footings, use concrete mix (contains aggregate). For repair work less than 2 inches thick, use sand mix or repair mix (no coarse aggregate). For setting fence posts, use fast-setting concrete mix that requires no mixing - just pour the dry mix into the hole and add water.
- Store bags off the ground. Cement-based products absorb moisture from concrete floors. Store bags on a pallet or 2x4s and cover with a tarp. Moisture-damaged bags have hard lumps and reduced strength - do not use them for structural work.
Worked Examples
Small Concrete Repair - Patio Section
A 4 x 4 ft damaged patio section, 4 inches thick, using 60 lb pre-mixed concrete bags.
- →Calculate volume: 4 ft x 4 ft x 0.33 ft (4 in) = 5.28 cu ft.
- →A 60 lb pre-mixed concrete bag yields about 0.45 cu ft. Base bags: 5.28 / 0.45 = 11.7 bags. Round up to 12 bags.
- →Add 10% waste for spillage and leveling: 12 x 1.10 = 13.2, round up to 14 bags.
- →Mix in batches of 2 bags at a time. Allow 3-4 hours for the full pour with mixing and placing.
Result: 14 bags of 60 lb concrete mix for a 4 x 4 ft patio repair section.
For small repairs, buy one extra bag beyond your calculated number. Partial bags can be stored dry for future repairs if kept in a sealed bucket. Match the color and brand for a visually consistent patch.
Fence Post Holes
8 fence posts with 10-inch diameter holes, 30 inches deep, using 80 lb fast-setting concrete mix.
- →Calculate volume per hole: π x (10 in / 2 / 12) x (10 in / 2 / 12) x (30 in / 12) = 3.14 x 0.417 x 0.417 x 2.5 = 1.36 cu ft per hole.
- →Total volume: 8 holes x 1.36 cu ft = 10.88 cu ft.
- →An 80 lb fast-setting concrete bag yields about 0.6 cu ft. Base bags: 10.88 / 0.6 = 18.1 bags. Round up to 19 bags.
- →Add 15% waste for over-dig and spillage around holes: 19 x 1.15 = 21.9, round up to 22 bags.
Result: 22 bags of 80 lb fast-setting concrete mix for 8 fence post holes.
Fast-setting concrete is ideal for fence posts because it sets in 20-40 minutes and requires no mixing - pour the dry mix directly into the hole, add water, and tamp. This reduces waste and speeds up installation. One 80 lb bag per post hole is a common rule of thumb for 10-inch diameter holes at 30 inches deep.
Small Slab Using Bagged Mix
A 6 x 8 ft garden shed slab, 4 inches thick, using 80 lb pre-mixed concrete bags.
- →Calculate volume: 6 ft x 8 ft x 0.33 ft = 15.84 cu ft.
- →An 80 lb bag yields 0.6 cu ft. Base bags: 15.84 / 0.6 = 26.4 bags. Round up to 27 bags.
- →Add 10% waste for leveling and edge forms: 27 x 1.10 = 29.7, round up to 30 bags.
- →Order 30 bags. At 45-60 minutes per 3-bag batch, allow a full day for mixing and placing.
Result: 30 bags of 80 lb concrete mix for a 6 x 8 ft shed slab.
For slabs over 15 cu ft, consider whether ready-mix delivery is more practical. A 6 x 8 slab at 4 inches is about 0.6 cu yd - most ready-mix companies have a 1 cu yd minimum. If you can use the extra concrete for a walkway or pad, delivery may be worth the cost and will save hours of mixing labor.
Mortar Mix for a Block Wall
Building a 20 ft long, 4 ft high concrete block wall using 8x8x16 standard CMU blocks, mixing mortar from 80 lb Type S mortar bags.
- →Calculate wall area: 20 ft x 4 ft = 80 sq ft. At 1.125 blocks per sq ft (including mortar joints), total blocks: 80 x 1.125 = 90 blocks.
- →Each block requires about 0.04 cu ft of mortar. Total mortar volume: 90 x 0.04 = 3.6 cu ft.
- →An 80 lb bag of mortar mix yields about 0.7 cu ft of mixed mortar. Base bags: 3.6 / 0.7 = 5.1 bags. Round up to 6 bags.
- →Add 15% waste for mixing and buttering: 6 x 1.15 = 6.9, round up to 7 bags.
Result: 7 bags of 80 lb Type S mortar mix for a 20 x 4 ft block wall (90 blocks).
Type S mortar is recommended for below-grade and load-bearing block walls. For above-grade non-load-bearing walls, Type N is sufficient and more economical. Use our Brick Calculator and Concrete Block Calculator to verify block counts before ordering mortar.
Waste Factors by Material
Always order more than your exact calculated quantity. Material suppliers typically do not accept returns on cut or opened materials. The waste factor accounts for cuts at walls, corners, defects, and installation error.
| Bag Size | Yield (cu ft) | Common Use | Related Tool |
|---|---|---|---|
| 40 lb bag | 0.30 cu ft | Small repairs, step repairs, or mixing by hand in a bucket | Concrete Calculator |
| 50 lb bag | 0.38 cu ft | Medium walkway repairs, small footings | Concrete Calculator |
| 60 lb bag | 0.45 cu ft | General DIY - slabs, patios, walkways up to 15 bags | Concrete Calculator |
| 80 lb bag | 0.60 cu ft | Most common for larger DIY projects. Best value per lb. | Concrete Calculator |
| 94 lb Portland cement | 1.0 cu ft (cement) | For custom 1:2:3 mixing with sand and gravel. Makes approx. 4.5 cu ft concrete. | Sand Calculator |
Square Footage by Project Type
| Project | What to Measure | Unit | Key Note | Related Tool |
|---|---|---|---|---|
| Fence posts | Number of posts, hole diameter, hole depth | Bags per post | Use fast-setting concrete; one 80 lb bag per post hole (10 in x 30 in) | Gravel Calculator |
| Small patio slab (under 100 sq ft) | Length x width x thickness (4 in standard) | Cubic feet or cubic yards | Bagged mix is practical under 2 cu yd; ready-mix for larger areas | Concrete Calculator |
| Walkway repair | Length x width x depth of repair area | Cubic feet | Use repair mix or sand mix for depths under 2 in; standard concrete mix for deeper patches | Concrete Calculator |
| Masonry / block wall | Wall area in sq ft, block type | Bags of mortar mix | Use Type N for above-grade walls, Type S for below-grade or retaining walls | Concrete Block Calculator |
| Footings (foundation) | Length x width x depth of trench | Cubic yards | Footings larger than 2 cu yd should use ready-mix; bagged mix for small shed footings | Concrete Calculator |
Reference Table
| Project Size | Volume | 80 lb Bags | 60 lb Bags | 94 lb Bags |
|---|---|---|---|---|
| 4x4 slab (4 in) | 0.2 cu yd | 9 bags | 11 bags | 8 bags |
| 10x10 slab (4 in) | 1.23 cu yd | 55 bags | 69 bags | 49 bags |
| 12x12 slab (4 in) | 1.78 cu yd | 80 bags | 100 bags | 71 bags |
| 20x20 slab (4 in) | 4.94 cu yd | 223 bags | 279 bags | 198 bags |
How We Calculate
Volume Calculation
The calculator starts by converting all project dimensions to cubic feet. For slab work: Length (ft) x Width (ft) x Depth (ft). For post holes: π x (Hole Radius in ft) x (Hole Radius in ft) x Depth (ft). For footings: cross-sectional area (sq ft) x length (ft). The calculator supports both imperial (feet/inches) and decimal foot inputs.
Bag Yield Conversion
Bag yields are based on industry-standard manufacturer specifications. Pre-mixed concrete bags use a fixed water-to-mix ratio and produce consistent yields per bag. The calculator divides the total project volume by the selected bag yield and applies the waste multiplier. Bulk Portland cement calculations assume a 1:2:3 mix ratio (cement : sand : gravel) which is the most common specification for residential concrete in the US.
Waste Factor Application
Waste factors account for multiple real-world variables: over-excavation or uneven subgrades (1-3%), spillage during mixing and transport (2-4%), material left in the mixer or wheelbarrow (1-2%), and form misalignment or bulging (1-2%). The 10% standard waste factor is consistent with recommendations from the Portland Cement Association and the National Ready Mixed Concrete Association.
Rounding and Ordering
The calculator always rounds up to the nearest full bag because partial bags cannot be purchased. This rounding typically adds 1-2 bags above the calculated need, which effectively provides a small additional waste buffer. The final number is a planning estimate - actual job site conditions may vary. Always discuss your estimated volume with your material supplier before ordering.
References and Data Sources
Portland Cement Association (PCA) - Concrete Design and Construction Guide
The PCA provides authoritative guidance on concrete mix design, bag yield specifications, and placement best practices for residential and commercial construction. Their Design and Control of Concrete Mixtures is the standard reference for mix ratios, water-cement ratios, and material properties used in North America. Portland Cement Association, 2025 Edition, www.cement.org.
National Ready Mixed Concrete Association (NRMCA) - Concrete Volume Estimating
NRMCA publishes standardized methods for estimating concrete quantities, including waste factors for various types of construction. Their guidelines are used by professional estimators and concrete producers across the United States. The 5-15% waste factor range recommended by NRMCA is incorporated into this calculators default settings.
ASTM C150 / C150M - Standard Specification for Portland Cement
Defines the chemical and physical requirements for Portland cement types (I, II, III, IV, V) used in US construction. Type I is the general-purpose cement used in most residential applications. This standard ensures consistency across manufacturers and is referenced by all major building codes.
International Residential Code (IRC) - Chapter 4: Foundations and Footings
The IRC establishes minimum requirements for concrete foundations, footings, and slabs-on-grade, including minimum thickness, reinforcement requirements, and concrete strength specifications. Local building codes adopt the IRC with amendments that may affect material requirements for specific regions.
All references are used for general estimation guidance only. BuildCalcHub does not claim certification, endorsement, or partnership with any listed organization. Always consult a licensed professional for your specific project requirements.
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