Example data table
| Use case | Typical parts (Vermiculite : Other components) | Why it works |
|---|---|---|
| Seed Starting | 1 : Coco/Peat 1 : Compost 1 | Balanced moisture and air spaces for uniform germination. |
| Light Potting Blend | 1 : Coco/Peat 2 : Compost 1 : Perlite 1 | Lower bulk density with steady water availability for roots. |
| Rooting Cuttings | 2 : Coco/Peat 1 | High moisture retention while keeping structure open. |
| Soil Amendment | 1 : Soil 4 : Compost 1 | Improves tilth and moisture buffering in beds. |
| Container Drainage Boost | 1 : Soil 3 : Bark 1 : Compost 1 | Better aeration and reduced compaction for containers. |
Formula used
This calculator splits a target batch volume by the share of each component’s parts.
- BatchVolume = TargetVolume × (1 + Allowance% / 100)
- TotalParts = VermiculiteParts + Σ(OtherParts)
- ComponentVolume = BatchVolume × (ComponentParts / TotalParts)
- VermiculiteWeight ≈ VermiculiteVolume(m³) × BulkDensity(kg/m³)
- WaterHolding ≈ VermiculiteWeight(kg) × Multiplier
How to use this calculator
- Pick a recipe preset, or choose Custom.
- Enter your total mix volume and the unit you measure with.
- Set a settling allowance if you expect compaction or spillage.
- Enter parts for vermiculite and at least one other component.
- Click Calculate Ratio to view volumes above the form.
- Use Download CSV or Download PDF for a batch sheet.
How vermiculite changes a growing medium
Vermiculite is a heat-expanded mineral that holds water in layered plates while keeping mixes airy. It buffers moisture, improves seed-to-media contact, and reduces crusting in trays. Fine grades suit germination; medium grades fit potting blends. A practical planning density is 80–150 kg/m³.
Selecting ratios for different gardening goals
Raise vermiculite share when uniform moisture matters most. For seeds, 1:1:1 (vermiculite:coco/peat:compost) is a reliable baseline. For cuttings, 2:1 (vermiculite:coco/peat) supports hydration without collapsing structure. For containers, keep vermiculite near 15–30% of total parts and balance nutrients with compost.
Turning “parts” into consistent measurements
A “part” is any repeatable scoop: cup, trowel, bucket, or jug. The calculator converts part shares into the exact volumes needed for your target batch size and unit. Use the same container for every ingredient and level it the same way to keep batches consistent.
Allowance, compaction, and batching accuracy
Dry components settle during mixing and transport. An allowance of 3–7% usually covers settling; 8–12% helps when screening compost or filling many small pots. Mix dry first, then wet gradually. Over-wetting reduces porosity; under-wetting leaves dry pockets that weaken germination.
Example data you can replicate quickly
Use the examples below as starting points. After calculating, export the batch sheet so you can repeat the same ratios across sowings and seasons, even when you change container sizes. Track watering response for a week, then adjust parts by 0.25–0.5 for fine tuning.
| Scenario | Target volume | Allowance | Parts (V : A : B) | Notes |
|---|---|---|---|---|
| Seed tray batch | 12 L | 7% | 1 : 1 : 1 | Even moisture; sifted compost recommended. |
| Light potting mix | 20 L | 5% | 1 : 2 : 2 | Adapt by adding perlite if drainage is slow. |
| Cuttings propagation | 8 L | 6% | 2 : 1 : 0 | Avoid rich compost for sensitive cuttings. |
FAQs
1) What does “1 part” mean in this calculator?
One part is any consistent volume measure, like a cup or bucket. The calculator scales those parts into exact volumes for your target batch size.
2) Should I use fine or medium vermiculite?
Fine vermiculite works best for seeds and plug trays. Medium grades suit pots and raised beds where you want more structure and air space.
3) Why add an allowance percentage?
Ingredients settle and some mix stays on tools. Allowance adds extra loose volume so the finished filled volume matches your target after mixing.
4) How are vermiculite weight and water-holding estimated?
Weight uses volume converted to m³ multiplied by bulk density. Water-holding uses the weight multiplied by your chosen multiplier. Real values vary by grade and moisture.
5) Can I replace vermiculite with perlite at the same ratio?
Not exactly. Perlite drains faster and holds less water. If swapping, reduce moisture expectations and recalibrate parts to match your watering schedule and plant needs.
6) What if my mix stays too wet?
Reduce vermiculite parts, add coarse materials like bark or perlite, and avoid compressing the mix. Confirm containers drain freely and water lightly after sowing.
7) What if seedlings dry out too quickly?
Increase vermiculite slightly, use a finer grade, and cover trays until germination. Water gently and evenly to prevent channels and dry pockets.