In This Guide
Why Gravity Systems Are Ideal for Off-Grid
No pump means no electricity dependency, no moving parts to fail, and near-silent operation. The physics are simple: water in a tank elevated above your fixtures flows by gravity. Pressure is determined by the height difference between the water surface and the fixture.
A tank elevated 2.31 feet above a fixture generates 1 PSI of pressure. Twenty feet of elevation = 8.7 PSI (adequate for gravity shower heads and sink faucets). Forty-six feet of elevation = 20 PSI (comfortable shower pressure). If your land has natural terrain elevation, this is often the most reliable and lowest-maintenance water system possible.
Calculating Your Water Pressure
Formula: Pressure (PSI) = Height (ft) ÷ 2.31
| Tank Height | Working Pressure | Suitability |
|---|---|---|
| 10 ft | 4.3 PSI | Gravity fill only (toilet tanks, stock troughs) |
| 20 ft | 8.7 PSI | Minimal shower, sink faucets |
| 35 ft | 15.2 PSI | Comfortable shower, all faucets |
| 46 ft | 20 PSI | Good all-round household pressure |
| 70 ft | 30 PSI | Equivalent to low-end municipal pressure |
Shower Minimum
Most shower heads require 8 PSI to function; 15–20 PSI for a comfortable experience. If you can only achieve 8–10 PSI, use a low-pressure rain shower head designed for gravity systems ($20–$40) — they work well at lower pressure.
Tank Sizing
Estimate daily household consumption first:
- Drinking/cooking: 1–2 gal/person/day
- Dish washing: 2–4 gal/day
- Bathing/shower: 5–15 gal/person/day
- Toilet flushing: 1.28–1.6 gal/flush × flushes/day
- Total estimate: 10–30 gal/person/day for conservation-minded off-grid use
Size your tank for 3–5 days of autonomy in case your fill source is interrupted.
| Tank Size | Type | Approx Cost |
|---|---|---|
| 250 gallon HDPE | New polyethylene | $120–$160 |
| 500 gallon HDPE | New polyethylene | $200–$280 |
| 1,000 gallon polyethylene | New | $350–$500 |
| 275–330 gallon IBC tote | Used (food-grade only) | $50–$150 |
Pipe Sizing for Adequate Flow
Pipe diameter determines maximum flow rate. Pressure alone doesn’t guarantee flow — undersized pipe creates friction losses that drop effective pressure at the fixture.
| Pipe Diameter | Max Flow Rate (at 15 PSI) | Recommended Use |
|---|---|---|
| 1/2″ | 3.5 GPM | Single fixture |
| 3/4″ | 7.5 GPM | 2–3 simultaneous fixtures |
| 1″ | 15 GPM | Whole-house main |
Use polyethylene (poly) pipe for buried runs — flexible, frost-resistant, and UV-stable when buried. For whole-house systems: 1″ main, 3/4″ sub-mains, 1/2″ branches to individual fixtures.
Tank Placement and Stand Construction
Options: hillside tank (free elevation from terrain — ideal if available), elevated stand (built elevation), or roof tank (requires structural engineering). For a stand build:
- 4×4 treated lumber posts, cross-braced with 2×6 diagonal bracing
- Double 2×8 beams across the top platform
- Weight calculation: 500 gal × 8.34 lb/gal = 4,170 lbs — over-engineer the stand deliberately
- Cover the tank: algae grows in sunlit tanks; UV degrades untreated HDPE over time
- Always include an overflow pipe directed downhill, away from foundations
Filtration Integration
A gravity system needs filtration staged in line:
- Coarse sediment filter (50 micron) at the tank outlet
- Carbon block filter at the distribution tee inside the building
- Gravity filter (Big Berkey) teed off before the kitchen sink for drinking/cooking
- UV purifier (optional) after sediment and carbon stages if your source has biological contamination
First-Flush Diverter for Rainwater
If filling from rain catchment, a first-flush diverter ($30) routes the first 10–20 gallons of roof runoff (which carries dust and debris) away from the tank. This alone dramatically extends filter life and improves water quality.
Step-by-Step Installation Checklist
- Test your water source and determine filtration requirements
- Calculate daily consumption; size tank for 3–5 days autonomy
- Choose tank location; measure actual elevation above lowest fixture
- Calculate working pressure at lowest fixture (height ÷ 2.31)
- Size main pipe for peak simultaneous flow (see table above)
- Build or install tank stand — engineer for full tank weight
- Install tank with inlet, overflow, outlet with shutoff valve, and drain
- Run main distribution pipe from tank to building
- Install sediment filter at tank outlet and carbon filter at distribution point
- Tee off for drinking water filtration (Big Berkey or other)
- Fill tank, open all fixtures one at a time, check for leaks
- Verify pressure at each fixture with an inline gauge (~$10)
Where to Go Next
- Water Filtration Guide — choosing the right filter for your source
- Rainwater Harvesting — filling your tank from roof catchment
- Big Berkey Review — our recommended gravity filter for drinking water
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