Enter slab details
Example data table
Use these examples to verify your inputs and expectations.
| Scenario | Inputs | Base Area | Adjusted Area (5% waste) |
|---|---|---|---|
| Garden patio (rectangle) | Length 12 ft, Width 10 ft | 120.00 ft² | 126.00 ft² |
| Fire pit pad (circle) | Diameter 8 ft | 50.27 ft² | 52.78 ft² |
| Shed corner (L-shape) | Outer 16×12 ft, Cutout 6×4 ft | 168.00 ft² | 176.40 ft² |
Formula used
- Rectangle: Area = Length × Width, Perimeter = 2(Length + Width).
- Circle: Area = πr², Circumference = 2πr (or πD).
- Right triangle: Area = ½ × Base × Height. Perimeter assumes right angle: Base + Height + √(Base² + Height²).
- Equilateral triangle: Area = (√3/4) × Side², Perimeter = 3 × Side.
- Trapezoid: Area = ½ × (Base1 + Base2) × Height. Perimeter assumes isosceles legs: Leg = √(((Base2−Base1)/2)² + Height²).
- L-shape: Area = (OuterLength × OuterWidth) − (CutoutLength × CutoutWidth). Perimeter equals the outer rectangle perimeter for a corner cutout.
- Waste: AdjustedArea = BaseArea × (1 + Waste%/100).
- Volume (optional): Volume = AdjustedArea × Thickness.
How to use this calculator
- Select your slab shape and choose feet or meters.
- Enter the required dimensions for that shape.
- Set a waste allowance for cuts and edging.
- Optional: Add thickness to estimate concrete volume.
- Press Calculate to show results above the form.
- Download CSV or PDF to save your plan.
Purpose and scope
Accurate slab area is the first control point for garden hardscape work. It determines concrete volume, base material coverage, reinforcement spacing, and finishing labor. This calculator is designed for patios, shed pads, walkways, outdoor kitchens, and compact equipment pads where clear takeoffs prevent ordering delays. Use the unit outputs to communicate with suppliers. Record dimensions before excavation to verify.
Choosing the right shape
Selecting a matching geometry reduces measurement mistakes on site. Rectangles and squares fit straight form boards and paver bases. Circles are common around fire pits, planters, and trees. Triangles and trapezoids model tapered paths or angled corners. L‑shapes represent pads that wrap posts or meet existing structures. If a slab includes multiple sections, run separate calculations and sum the adjusted areas for a clean material schedule.
Waste and overage planning
Waste is shown as a percentage so you can document your allowance. A 5–10% buffer often covers saw cuts, rounded edges, small layout changes, and minor subgrade corrections. Increase the buffer for decorative borders, complex joints, hand‑mixed pours, or tight access where spillage and rework are more likely. Keeping the buffer explicit also improves quote comparability across competing bids.
From area to concrete volume
When thickness is entered, the tool converts adjusted area into cubic feet, cubic yards, and cubic meters. This supports selecting ready‑mix delivery versus bagged mix for smaller garden projects. Bag counts use typical yield assumptions, so verify the exact yield printed on your product and consider consolidation loss during placement. For sloped slabs, calculate average thickness or model the slab in segments to avoid underestimating volume.
Perimeter for garden detailing
Perimeter helps estimate edging, form lumber, expansion joint filler, sealant, and border treatments. For triangles and trapezoids, perimeter uses standard leg assumptions, so confirm side lengths if the layout is irregular. Use perimeter with your desired control‑joint spacing to approximate joint length for saw cutting. Exported CSV and PDF summaries keep measurements consistent for bids, material lists, and supplier requests.
FAQs
1) What waste percentage should I use?
Use 5–10% for typical garden slabs. Increase it for curved forms, decorative edges, hand mixing, or tight access where spillage and rework are more likely.
2) Can I calculate multiple slab sections?
Yes. Run each section as its own shape, then add the adjusted areas. This method is clearer than forcing one irregular shape and helps produce accurate material and labor takeoffs.
3) How is concrete volume estimated?
Volume equals adjusted area times thickness. The calculator converts thickness to meters, then reports volume in cubic meters, cubic feet, and cubic yards for ordering and comparison.
4) Are bag estimates exact?
No. Bag counts use typical yield values that vary by product, moisture, and compaction. Always confirm the stated yield on your mix packaging and consider buying an extra bag or two.
5) How does the L-shape option work?
It subtracts a rectangular cutout from an outer rectangle. Cutout dimensions must be smaller than the outer dimensions. This suits pads that wrap posts or avoid existing planters and footings.
6) Why might perimeter be approximate?
For triangles and trapezoids, perimeter uses standard assumptions about side lengths. If your layout has irregular sides, measure the actual edges and use that value for edging, forms, and joint materials.