Shed Base Calculator

1. Choose the base type: a concrete slab or a gravel pad.
2. Enter the base length and width in feet, a little larger than the shed, and the thickness or depth in inches.
3. Set the number of bases and a 5 to 10 percent waste allowance.
4. Read the result: cubic yards to order, plus bags and weight for concrete, or tons for gravel.

Here is what each result line means:

A shed base calculator estimates the material for the base a shed sits on, whether that is a poured concrete slab or a compacted gravel pad. The base keeps the shed level, dry and off the ground, and the two types are measured and ordered differently.

A concrete slab is a permanent, level floor, ordered by the cubic yard or in bags. A gravel pad is a bed of compacted crushed stone, ordered by the cubic yard or the ton, that drains well under a timber-floored shed. The tool sizes either from the base dimensions. For the concrete mix itself, use the concrete mix calculator or the cement calculator, and for mix water the water cement ratio calculator.

The volume is the same for both base types: length times width times thickness, divided by 27 for cubic yards. What changes is how the quantity is reported, by bags for concrete or by tons for gravel.

1. Multiply length by width by thickness in feet, and by the number of bases, for the cubic feet.
2. Add the waste allowance, then divide by 27 for cubic yards.
3. For concrete, divide cubic feet by the bag yield for bags, and use 150 pounds per cubic foot for the weight.
4. For gravel, multiply the cubic yards by about 1.5 for the weight in short tons.

Suppose you are building a 10 by 8 foot base at 4 inches, with 10 percent waste.

Calculation: the area is 80 square feet. Volume = 80 x 0.333 = 26.67 cu ft, or 0.99 cu yd. Adding 10 percent waste gives 29.33 cu ft, which is 1.09 cu yd. As concrete that is about 49 eighty-pound bags and 2.2 short tons; as gravel it is about 1.6 short tons of crushed stone.

The lesson: the same hole holds the same volume either way, but a slab is dozens of bags of concrete while a pad is a couple of tons of stone to compact.

Base Size

Make the base a little larger than the shed footprint so the walls are not at the very edge. That margin adds to the material, so size the base, not just the shed.

Thickness or Depth

A 4 inch concrete slab suits most sheds; a gravel pad is often 4 to 6 inches of compacted stone. Depth multiplies the whole area, so it moves the quantity the most.

Sub-base and Compaction

A gravel pad is compacted in layers and settles, and a slab needs a firm sub-base. Both reasons mean the loose or over-dug volume beats the finished geometry, which the waste allowance covers.

Ground and Drainage

Soft or wet ground may need a deeper base or a membrane. That changes the depth you enter rather than the formula.

Both make a sound shed base; they differ in cost, drainage and permanence. A gravel pad is cheaper and drains freely, while a concrete slab is a permanent, level floor.

Pick the base type above and the tool reports bags for concrete or tons for gravel, so you can compare what each actually takes.

Ordering the Base Material

Size the concrete or gravel in cubic yards and add waste before you order, so you neither run short nor over-buy.

Choosing the Base Type

Compare the bags of concrete against the tons of gravel for the same base to weigh cost, effort and drainage.

Planning Delivery

The weight tells you whether the gravel comes by the ton or the slab by ready-mix, and how to get it to the site.

1. Sizing the Shed, Not the Base

The base should be a little larger than the shed. Using the shed footprint leaves no margin and under-orders.

2. Mixing Up Feet and Inches

Entering the depth in feet instead of inches shrinks the result twelvefold. Keep length and width in feet and depth in inches.

3. Forgetting Compaction and Waste

Gravel settles as it is compacted and a slab sub-base is rarely even. Without a waste allowance you run short.

4. Trusting the Gravel Tonnage Exactly

Stone density varies from about 1.4 to 1.7 tons a cubic yard. The tool uses about 1.5; confirm the exact figure with your supplier for a precise order.

5. Skipping the Sub-base Under a Slab

A concrete shed slab still needs a firm, compacted base under it. The slab volume does not include that stone layer; size it separately.

The arithmetic is exact; the usefulness depends on the dimensions and the waste you enter, and the gravel tonnage uses a typical density.

It depends on the size. A 10 by 8 foot base at 4 inches needs about 1.09 cubic yards with waste, roughly 49 eighty-pound bags. Enter your base size above for the exact figure, and make the base a little larger than the shed.

For a 10 by 8 foot pad at 4 inches, about 1.09 cubic yards, which is roughly 1.6 short tons of crushed stone at a typical density. Gravel is ordered by the cubic yard or the ton; confirm the exact tonnage with your supplier.

A concrete slab gives a hard, level, permanent floor and suits heavy sheds and workshops. A gravel pad is cheaper, drains freely and suits a timber-floored shed on damp ground. Pick the base type above to compare what each takes.

A concrete slab is commonly 4 inches, and a gravel pad is often 4 to 6 inches of compacted stone. Heavier sheds or soft ground may need more. Follow the shed maker instructions or local practice; this tool estimates the material.

A little larger than the shed footprint, so the walls do not sit at the very edge and water drains away. Enter the base size, not the shed size, so the margin is included in the material.

A 10 by 8 foot, 4 inch concrete slab weighs about 2.2 short tons; the same pad in gravel is about 1.6 tons. Either way the ground under it must be firm and compacted to carry the load without settling.

Yes, a concrete slab still needs a firm, compacted sub-base, usually of crushed stone. This tool sizes the slab or the pad; size a sub-base layer under a slab separately from its own depth and area.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• QUIKRETE Concrete Mix 1101 data sheet (QUIKRETE; bag yields 0.60, 0.45 and 0.30 cu ft).
• Inch Calculator, stone and gravel weight (crushed stone about 1.4 to 1.7 short tons per cubic yard).
• National Ready Mixed Concrete Association (NRMCA; ready-mixed concrete and ordering).
• American Cement Association (American Cement Association; what concrete and cement are).