Planter Box Volume Calculator

How volume is calculated

• Rectangular: V = L × W × H
• Round: V = π × (D ÷ 2)² × H
• Tapered (rectangular frustum): V = H/3 × (A_top + A_bottom + √(A_top×A_bottom))
• Drainage layer: Subtracted before fill and allowance steps.
• Filled soil: V_fill = V_net × (Fill% ÷ 100)
• Purchase allowance: V_buy = V_fill × (1 + Allowance% ÷ 100)

Steps for accurate soil planning

1. Select the planter shape and measurement units.
2. Choose Inner for soil space, or Outer and enter thickness values.
3. Enter dimensions and the number of boxes.
4. Add a drainage thickness if you use gravel or lightweight fill.
5. Set fill level and allowance to match your planting style.
6. Press Calculate and export results if needed.

Sample planter volumes

Why accurate soil volume matters

Planter boxes rarely match “bag math” in real life. A box that looks small can still exceed 150 liters once depth is included. Correct volume prevents underfilling, uneven plant growth, and repeated store runs. This calculator standardizes measurements, converts units instantly, and reports total soil for multiple boxes. That consistency is valuable when you are comparing mixes, calculating delivery loads, or budgeting a whole patio refresh.

Understanding fill level and settlement

Many gardeners do not fill to the rim. A 90–95% fill level leaves watering headspace and reduces spillover. Settlement allowance adds extra material for compaction after the first few waterings. For most potting blends, 5–15% is realistic, depending on moisture and particle size. Using both settings together produces a purchase figure that better matches what you actually need on day one.

Drainage layers and lightweight fillers

If you add gravel, perlite, LECA, or a coarse layer at the bottom, that space should not be counted as soil. Enter the drainage thickness to subtract it from the internal volume before applying fill and allowance. For raised boxes, a 2–5 cm drainage layer is common, while deep planters may use more. Accurate subtraction keeps roots in the intended soil depth and avoids buying excess mix.

Choosing units and comparing outputs

Projects often mix metric and imperial measurements. The unit selector converts mm, cm, m, inches, and feet into a single internal model, then outputs liters, cubic feet, cubic yards, cubic meters, and US gallons. Liters are ideal for bag planning, cubic feet help when comparing soil brands, and cubic yards or cubic meters support bulk orders. The multi-unit report reduces conversion errors during purchasing.

Planning purchases: bags versus bulk

Bagged soil is convenient but can cost more for larger builds. A common bag size is 40 liters, and the calculator rounds up to full bags so you can load a cart confidently. Bulk pricing is better for high volumes, but suppliers may have delivery minimums. Use the bulk unit selector to estimate cubic yards or cubic meters, then compare costs against bag totals for the clearest decision.

1) Should I measure inner or outer dimensions?

Use inner dimensions for soil space. If you only have outside measurements, choose Outer and enter wall and bottom thickness so the calculator estimates usable soil volume.

2) What fill level should I pick?

Most planters work well at 90–95% fill. It leaves room for watering and mulch, and helps avoid spillover when you top-dress or mix compost.

3) How much settlement allowance is normal?

A 5–15% allowance is typical. Light mixes settle less; dry mixes and fine particles settle more after repeated watering and root growth.

4) Do I really need a drainage layer?

Not always. Good drainage holes and a suitable mix are more important. If you do add a coarse layer, enter its thickness so soil estimates remain accurate.

5) Why does the tapered option change the result a lot?

Tapered boxes are wider at the top and narrower at the base. The calculator uses a frustum volume method, which often produces less volume than a simple rectangular estimate.

6) Can I export results for my records?

Yes. After you calculate, use the Download CSV or Download PDF buttons. Exports include your inputs and the purchase volumes in multiple units.