How much soil, what to mix, and whether your fertilizer fits your plants.
Raised Bed Dimensions
Length & width in feet, depth in inches — the way beds are usually sized.
Soil Mix Recipe
Compost
34% — nutrients & biology
5.4 ft³
Base (topsoil / coco coir / peat)
33% — body & moisture
5.3 ft³
Aeration (perlite / vermiculite)
33% — drainage & air
5.3 ft³
Soil Needed
16 ft³
411dry quarts
453litres
0.59cubic yards
≈ 11 standard 1.5 ft³ bags
Fertilizer Doses
scaled to your volume
Pick whichever one you have on hand — these are alternatives, not meant to be combined.
All-purpose organic (e.g. 4-4-4)16 cups
Worm castings top-up8 cups
Slow-release granular5.3 cups
Guideline rates: 1 cup all-purpose, ½ cup castings, ⅓ cup slow-release per cubic foot of mix.
Always confirm against your product's label — brands vary.
What are you growing?
Your fertilizer bag's N-P-K numbers
iThe P and K numbers on a US fertilizer bag aren't pure elemental phosphorus or potassium — they're phosphate (P₂O₅) and potash (K₂O) equivalents. That's the standard label convention, so a 10-10-10 bag has 10% N, 10% P₂O₅, and 10% K₂O by weight.
Match Check
iWe compare your bag's N/P/K balance against this plant's ideal ratio and flag whichever nutrient is furthest off — a simplified version of the scoring systems professional soil-test tools use, without needing lab data.
Decent match
Checking your bag against the plant's ideal ratio…
How much to apply
iBased on a general 0.2 lb of actual nitrogen per 100 sq ft per season — a reasonable vegetable-garden baseline — then scaled to your specific bag, since a 10-10-10 and a 30-0-0 need very different amounts to deliver that same nitrogen.
Per square foot— g
Per 100 sq ft bed— lbs
Roughly— tbsp / sq ft
How Much Soil Do You Need?
Multiply length × width × depth. A classic 4×4 ft raised bed filled 12 inches deep holds 4 × 4 × 1 = 16 cubic feet of soil — about 11 standard 1.5 ft³ bags. Round pots use the circle formula: π × radius² × depth.
For raised beds
Switch to "Bed," then enter your bed's inner length and width in feet, and its depth in inches — for example, 8 ft long × 4 ft wide × 12 in deep. You'll get the total volume in cubic feet (plus cubic yards, handy if you're having soil delivered by the truckload), a bag count based on standard bag sizes, and a topsoil/compost/aeration breakdown for whichever mix you've picked above.
For containers (pots or planters)
Switch to "Pot," then enter the diameter measured across the top of the pot, and its depth. You'll get the potting soil volume needed and a bag count, using the same 1.5 ft³ standard bag size.
A 3×6 ft bed, 10 inches deep
3 × 6 × (10 ÷ 12) = 15 cubic feet of blended soil. Split at a classic 60/30/10 ratio, that's:
That 60/30/10 split is exactly what our own Container Mix preset above uses — the same ratio shows up across most raised bed guides for good reason, it works.
Want the full mix breakdown, bed-depth guide, fertilizing tips, and NPK precision notes? See our Soil Mix Guide and FAQ.
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