Calculator Form
Formula Used
Rectangular bed volume: length × width × usable depth.
Round bed volume: 3.14159 × radius² × usable depth.
Usable depth: total depth − top gap.
Total raw volume: volume per bed × number of beds.
Final volume: raw volume × settlement factor × waste factor.
Bag count: final volume ÷ bag volume, rounded up.
Mix volume: final volume × normalized mix percentage.
How to Use This Calculator
- Select the bed shape.
- Choose the unit used for your measurements.
- Enter inside bed dimensions.
- Enter total depth and the empty top gap.
- Add settlement and waste allowances.
- Enter bag size and cost.
- Add your preferred soil mix percentages.
- Press the calculate button.
- Use CSV or PDF export for records.
Example Data Table
| Bed type | Dimensions | Depth | Quantity | Allowance | Approximate soil |
|---|---|---|---|---|---|
| Small herb bed | 4 ft × 2 ft | 1 ft | 1 | 10% | About 8.8 ft³ |
| Vegetable bed | 8 ft × 4 ft | 1 ft | 2 | 15% | About 73.6 ft³ |
| Round planter | 4 ft diameter | 1.5 ft | 1 | 10% | About 20.7 ft³ |
Raised Bed Soil Planning Guide
Raised bed planning matters
Raised beds make soil control easier. They also make filling mistakes costly. A bed that looks small can require many bags. Depth, shape, settlement, and top dressing change the final volume. This calculator helps you plan those details before buying soil.
Why volume changes
The basic volume is length times width times depth. Round beds use the area of a circle. That number gives the empty space inside the frame. Real soil often settles after watering. Compost can shrink as it breaks down. A small waste allowance also helps when the bed has corners, uneven ground, or a slightly bowed frame.
Using mix ratios
Many gardeners fill beds with a blended medium. A common mix may include topsoil, compost, and an aeration material. The best ratio depends on crop type, local soil, drainage, and budget. Heavy clay sites may need more compost and aeration. Dry sites may need more organic matter. The calculator normalizes the entered percentages, so the parts always match the final volume.
Bags or bulk
Bagged soil is simple for small beds. It is also easy to carry and store. Bulk soil is usually better for large projects. Use the cubic yard result when ordering delivery. Use the bag count when buying packaged material. Add the cost per bag to compare both choices. Always round up, because stores do not sell partial bags.
Practical filling tips
Leave a small top gap below the frame edge. This helps stop soil from washing out. Water the bed after the first fill. Then add more soil if the level drops. Avoid compacting the bed by walking on it. Raised bed roots need air as well as nutrients. For deep beds, some gardeners place rough organic material near the bottom. Do this only when it fits the crop plan.
Final check
Measure the inside dimensions, not the outside boards. Confirm the depth after subtracting mulch space. Review the waste percentage before purchase. The final estimate gives liters, cubic feet, cubic yards, bags, cost, and mix volumes. For best results, check labels on purchased products. Some bags list compressed volume. Others list loose volume. Keep receipts until the bed is filled and settled after watering for accurate records.
FAQs
How much soil do I need for a raised bed?
Multiply inside length by inside width by usable depth. Then add settlement and waste allowances. This calculator performs those steps and converts the answer into liters, cubic feet, cubic yards, and bag count.
Should I measure inside or outside dimensions?
Measure inside dimensions. Board thickness does not hold soil. Outside measurements can overestimate the needed amount, especially when thick lumber or blocks are used.
Why include a top gap?
A top gap helps prevent soil from spilling during watering. It also leaves space for mulch. The calculator subtracts this gap from the total depth before finding volume.
What settlement allowance should I use?
A 5% to 15% allowance is common for many loose mixes. Compost-rich blends may settle more. Freshly filled deep beds may also need a higher allowance after watering.
Can I calculate round raised beds?
Yes. Choose the round shape option. Enter the diameter and depth. The calculator uses the circle area formula, then applies quantity, settlement, and waste adjustments.
How does the bag count work?
The final soil volume is divided by the selected bag volume. The result is rounded up, because partial bags are not usually sold.
What do the mix percentages mean?
They divide the final volume into topsoil, compost, and aeration material. If the percentages do not total 100, the calculator normalizes them automatically.
Is bulk soil better than bags?
Bulk soil is often better for large beds. Bags are useful for small projects and easy transport. Compare cubic yards, bag count, and cost before buying.