Calculator
Example data table
| Pot type | Side / Top | Bottom | Height | Fill | Waste | Pots | Soil needed |
|---|---|---|---|---|---|---|---|
| Straight | 30 cm | — | 25 cm | 90% | 5% | 4 | ≈ 85.05 L |
| Tapered | 32 cm | 26 cm | 25 cm | 90% | 5% | 4 | ≈ 77.84 L |
Formula used
- Straight square pot (prism): V = s² × h, where s is inside side length and h is soil depth.
- Tapered square pot (frustum): V = (h / 3) × (A₁ + A₂ + √(A₁A₂)), where A₁ = a² (top area) and A₂ = b² (bottom area).
- Adjustments: Vfinal = V × fill% × (1 + waste%) × pots.
- Liters conversion: 1,000 cm³ = 1 L.
How to use this calculator
- Choose whether your pot is straight or tapered.
- Select your measurement unit, then enter inside dimensions.
- Set fill level to leave watering space at the top.
- Add a small extra percentage for settling and spillage.
- Enter how many pots you will fill, then calculate.
- Download your results as CSV or a printable PDF.
Why pot volume matters
Pot size is not just a label; it controls root space, watering frequency, and fertilizer buffering. Two square pots that look similar can hold very different soil volumes if their depths differ. For example, a 20 cm × 20 cm pot filled to 18 cm holds about 7.2 L, while the same footprint at 28 cm depth holds about 11.2 L—over 50% more. Use volume to standardize care across batches. That saves time and money.
How to measure a square pot correctly
Measure the inside width and length at the level where soil will sit, not the outside rim. For tapered pots, also measure the inside width at the bottom. Record depth as the planned soil depth, then reduce it by your top “watering space” (often 2–4 cm). Small measurement errors compound: being off by 1 cm on a 25 cm square changes area by roughly 8%, which directly changes volume.
Understanding unit conversions
This calculator converts common units to centimeters internally, then reports results in cm³, liters, and optionally US gallons. Remember: 1,000 cm³ equals 1 L, and 1 US gallon is about 3.785 L. If you buy soil by the cubic foot, 1 ft³ is about 28.3 L. Conversions help compare bag sizes and avoid overbuying.
Fill level, drainage, and realistic soil needs
Most gardeners do not fill pots to the brim. A fill level of 85–95% leaves room for watering and mulch. If you use a drainage layer, subtract its depth from the soil depth rather than guessing later. Potting mix also settles after watering; adding 3–8% extra is common for topping up. For multiple pots, small “waste” percentages prevent shortfalls during filling.
Planning purchases with quick checks
After calculating total liters, sanity-check against bag labels. A 40 L bag typically fills about five to six 7 L pots, or three to four 11 L pots, depending on compaction and amendments. If you blend compost or perlite, calculate the final mix volume first, then split by your recipe (for example 70/30). Keeping a consistent volume per pot improves growth uniformity.
FAQs
Should I use outside or inside measurements?
Use inside measurements because soil occupies the internal space. Outside dimensions include wall thickness and can overestimate volume, especially on thick plastic or ceramic pots.
How do I handle a tapered square pot?
Measure the inside width at the top soil line and at the bottom. The calculator uses a frustum method to estimate the average cross‑section, which is more accurate than averaging widths by eye.
What fill level should I choose?
Most containers work well at 90% fill, leaving room for watering and mulch. Use a lower fill level if you top‑dress heavily or expect significant settling after the first few irrigations.
How much extra soil should I add for settling?
Add 3–8% for most potting mixes. Coarse, chunky blends can need closer to 10% because they bridge and settle unevenly when watered.
Can I convert liters to soil bags quickly?
Yes. Divide total liters by the bag size in liters (for example 40 L). Then add one extra small bag if your waste percentage is above 5% or you plan to top up later.
Does this include space taken by roots or amendments?
It estimates the starting mix volume only. If you add large stones, biochar, or thick drainage layers, reduce soil depth accordingly, then recalculate so the result matches your actual fill plan.