Enter planter details
Example data table
| Scenario | Planter size | Count | Fill | Shrink | Adjusted volume | Bag size | Bags (rounded) |
|---|---|---|---|---|---|---|---|
| Vegetable bed | 48 in × 24 in × 12 in | 2 | 100% | 10% | 17.6 ft³ | 1.5 ft³ | 12 |
| Herb planter | 120 cm × 40 cm × 30 cm | 1 | 90% | 8% | 116 L | 40 L | 3 |
| Flower box | 36 in × 12 in × 10 in | 3 | 95% | 12% | 7.4 ft³ | 2.0 ft³ | 4 |
Formula used
- Planter volume = Length × Width × Depth × (Fill% ÷ 100)
- Total raw volume = Planter volume × Number of planters
- Adjusted volume = Total raw volume × (1 + Shrink% ÷ 100)
- Mix component volume = Adjusted volume × (Component% ÷ Sum of percents)
- Bag count = Adjusted volume ÷ Bag size (rounded up)
- Weight estimate = Adjusted volume × Bulk density
How to use this calculator
- Measure the inside length, width, and soil depth of your planter.
- Choose your unit system and enter the number of identical planters.
- Set a fill percentage if you are not filling fully.
- Add a shrink/settle factor to cover compaction after watering.
- Enter your mix percentages to split volumes for purchasing.
- Select a bag unit and size, then calculate and download results.
Planning soil purchases for raised planters
Measure inside dimensions, not the outside boards
Accurate soil volume starts with inside length, inside width, and intended soil depth. Lumber thickness and rim overhang can inflate outside measurements and overstate purchases. Measure at several points if walls bow. For tiered beds, calculate each tier separately and add totals.
Account for fill level and settling after watering
Many planters are not filled to the top because mulch, irrigation lines, or top dressing need space. The fill percentage adjusts the effective depth. After watering and a few days of gravity, mixes settle. A 5–15% shrink factor is common, and higher compost content can increase settling.
Use mix ratios to split topsoil, compost, and aeration
Balanced blends improve drainage, nutrient availability, and root oxygen. The calculator scales your topsoil, compost, and aeration percentages even if they do not total 100. This keeps proportions consistent while matching the final adjusted volume. Record the ratio you used for repeatable seasonal refills.
Estimate weight for handling and structural checks
Bulk density converts volume into weight, helping plan transport and load. Lightweight potting blends may be far lower than screened topsoil. If your planter sits on a deck or roof, verify allowable live loads, distribute weight with sleepers, and avoid waterlogging by ensuring drainage outlets remain clear.
Convert volume into bags and cost for procurement
Retail products are sold by liters or cubic feet. Selecting bag unit and size converts adjusted volume into a rounded-up bag count so you do not run short. Enter a price per bag to estimate budget. Consider buying an extra small bag for future topping and mixing corrections.
Track what you buy in the downloadable CSV, then compare it to what you actually used onsite. Differences often come from uneven bed bottoms, voids around irrigation, or intentional mounding for crops. When mixing components, blend on a tarp in small batches to keep ratios uniform and prevent dry pockets. Label leftover bags for quick top-ups later.
FAQs
What shrink factor should I start with?
Start with 10% for most blended soils. Use 5% for coarse, well-aerated mixes, and 15% for compost-heavy blends that compact after watering and rain.
Do I measure planter height or soil depth?
Use soil depth. If you leave space for mulch, edging, or drip lines, reduce depth or set a fill percentage so the calculated volume matches the soil you will add.
What if my mix percentages do not equal 100?
The calculator normalizes the values by their total. Your proportions stay the same, and volumes are scaled to match the adjusted soil requirement.
How do I choose a bulk density value?
Use supplier data when available. Light potting blends are often far lower than screened topsoil. If unsure, estimate and re-check by weighing a known bag volume.
Why does the bag count round up?
Bags are discrete units, and you cannot purchase partial bags. Rounding up reduces the risk of shortages caused by settling, uneven bottoms, or small measurement errors.
Can I use this for irregular or tapered planters?
Approximate the shape by splitting it into simple rectangles, then add volumes. For tapered beds, use the average of top and bottom widths and lengths for a practical estimate.