Calculator Inputs
Example Data Table
| Bed area | Climate | Mulch | Depth | Need | Weeks | Saved | Saved % |
|---|---|---|---|---|---|---|---|
| 12 sq m | Mild | Wood chips | 75 mm | 25 mm/wk | 26 | ~3,250 L | ~40% |
| 25 sq m | Hot | Straw | 60 mm | 30 mm/wk | 20 | ~6,200 L | ~37% |
| 150 sq ft | Arid | Plastic film | — | 35 mm/wk | 18 | ~5,900 L | ~55% |
Formula Used
This calculator treats weekly water needs as a depth applied over the bed. Since 1 mm over 1 m² equals 1 liter, it converts depth to volume with:
- NetNeed(mm/wk) = max(PlantNeed − EffectiveRain, 0)
- BaselineGross(mm/wk) = NetNeed ÷ (Efficiency/100)
- BaselineLiters/wk = Area(m²) × BaselineGross(mm/wk)
- MulchReduction = Base(Type) × DepthFactor × Climate × Wind × Soil × Coverage
- MulchedNet = NetNeed × (1 − MulchReduction)
- MulchedLiters/wk = Area(m²) × (MulchedNet ÷ (Efficiency/100))
- SavedLiters/wk = BaselineLiters/wk − MulchedLiters/wk
If you enter a water price, it converts your price to cost per liter and multiplies it by liters saved to estimate money saved. If you enter mulch price, it estimates material volume as Area × Depth × Coverage and provides payback.
How to Use This Calculator
- Enter your bed area and choose units.
- Set weekly plant water need and expected effective rainfall.
- Choose an irrigation efficiency that matches your system.
- Select climate, soil, wind, mulch type, depth, and coverage.
- Optional: add water cost and mulch price to estimate payback.
- Press Calculate Savings to see results above the form.
Baseline irrigation volume
The calculator starts with weekly plant demand, subtracts effective rainfall, and converts millimeters to liters. One millimeter across one square meter equals one liter, so a 12 m² bed needing 25 mm/week requires 300 liters net each week. If your area is in square feet, it converts to square meters before calculating volume.
Efficiency and delivery losses
Irrigation systems rarely deliver every liter to roots. Drip lines often run 80–95% efficient, while overhead sprinklers can fall near 60–75% in heat or wind. The tool divides net demand by your efficiency to estimate the gross volume you must apply. Improving distribution uniformity, fixing leaks, and watering at dawn can raise real efficiency without changing equipment.
Mulch reduction drivers
Mulch limits surface evaporation and reduces crusting. Organic chips at 50–100 mm commonly cut irrigation needs by roughly 30–55% under typical conditions, while plastic film can exceed 60% when edges are sealed. Coverage, depth, wind exposure, soil texture, and climate shift the final reduction. Sandy beds benefit because they dry quickly, and windy sites gain more because bare soil loses moisture faster.
Season totals and budgeting
Weekly savings scale quickly over a watering season. Saving 125 liters each week becomes 3,250 liters over 26 weeks. Add a water price per cubic meter, per 1,000 liters, per liter, or per gallon to translate that volume into seasonal cost savings. Use the CSV export to compare scenarios, like deeper mulch versus wider coverage, and keep a record for future planning.
Mulch quantity and payback
Material needs depend on area, depth, and coverage. A 10 m² bed mulched at 75 mm uses 0.75 m³ before settling. The calculator adds a waste factor for uneven spreading and compaction over time. If mulch costs 6,000 per m³ and your weekly water savings equal 150, payback is about 30 weeks, helping prioritize projects. Faster payback is typical when water is expensive or when irrigation efficiency is low. Track results after rain, and adjust weekly inputs accordingly.
FAQs
How do I estimate plant water need in mm per week?
Use local guidance or observe soil moisture. Many vegetables do well around 20–35 mm/week in warm weather. Reduce for shade, increase for new transplants, and confirm with a moisture probe.
What counts as effective rainfall?
Effective rainfall is the portion that reaches roots, not runoff or canopy drip loss. Light showers may be near zero in dense beds. Use a conservative estimate unless you measure infiltration.
Why does irrigation efficiency change my totals?
Lower efficiency means more water is applied to deliver the same root-zone amount. Overspray, evaporation, runoff, and uneven distribution reduce effective delivery, especially with sprinklers in wind.
Which mulch type saves the most water?
Plastic film often saves the most by sealing the surface, but it can limit air exchange. Wood chips and straw provide strong savings while improving soil structure and moderating temperature.
Can I use the custom reduction field?
Yes. If you have measured savings, enter your percentage and the tool will apply it, scaled by coverage. Keep values realistic; extremely high reductions can under-water plants.
How should I change my watering schedule after mulching?
Decrease irrigation gradually, then monitor leaf turgor and soil moisture below the mulch. Many gardens can cut frequency first, then duration. Re-check after heat waves or windy periods.