Size a composting toilet chamber by multiplying the number of users, the daily volume of cover plus faeces per person and the resting time, then dividing by the fraction of the chamber you allow to fill. For four people, 12 months of rest and urine diverted, that is 4 × 0.63-1.0 L × 365 ÷ 0.8 = 1.15 to 1.83 m³ per chamber, larger than the 1 m³ often built; if urine also goes into the chamber the same sum gives 3.7 to 4.4 m³, which is why it is diverted.
Part of Composting Toilet Systems
Chamber sizing sets the internal volume of each vault in a batch composting toilet from the number of users and the time the waste must rest. A chamber that is too small fills before the other one has finished resting.
The Chamber Must Fill In As Long As The Other Rests
In a two-chamber system the sealed chamber rests while the active one fills, so the fill time of one chamber equals the rest time of the other (Dual-Chamber (Batch) Design). To guarantee the parent page’s year of rest, each chamber must take at least 12 months to fill.
V (gross, m³) = users × litres per person per day × days of fill ÷ fill fraction ÷ 1,000
The Inputs, And Where Each Comes From
| Input | Value | Source |
|---|---|---|
| Cover per person per day | 0.5-0.75 L | Composting Toilet |
| Faeces per person per day | 0.13-0.25 L | Recalled published range of wet faeces, about 130-250 g per day at roughly 1 kg per litre; not re-read |
| Urine per person per day, if undiverted | about 1.4 L | Recalled from the same literature; not re-read |
| Fill fraction before sealing | 0.8 | The first-month page seals a vault at about 80% full |
| Rest time | 12 months | Parent page |
Solids plus cover per person: 0.5 + 0.13 = 0.63 L to 0.75 + 0.25 = 1.0 L per day. Treat this as an estimate, then measure your own (see below).
Worked Example: Four People, Two Chambers, Urine Diverted
- Days of fill = 365.
- Usable volume needed = 4 × 0.63 × 365 = 920 L to 4 × 1.0 × 365 = 1,460 L.
- Gross volume = 920 ÷ 0.8 = 1,150 L = 1.15 m³ to 1,460 ÷ 0.8 = 1,825 L = 1.83 m³.
A chamber of 1.15-1.83 m³ is, for example, 1.5 m × 1.2 m × 1.0 m = 1.8 m³. The parent page’s 1 m³ chamber holds 0.8 m³ usable and fills in 800 ÷ 1,460 × 12 = 6.6 months to 800 ÷ 920 × 12 = 10.4 months.
If Urine Goes Into The Chamber
Per person = 0.63 + 1.4 = 2.03 L to 1.0 + 1.4 = 2.4 L per day, so for four people over 365 days, 2.03 × 4 × 365 ÷ 0.8 = 3.7 m³ to 2.4 × 4 × 365 ÷ 0.8 = 4.4 m³. Nearly all of that extra is liquid, which does not compost; it makes the pile wet, airless and slow. Divert it (Urine-Diverting Systems).
Gross Volume Scales Directly With Household Size
Two chambers, 12 months of rest, urine diverted:
| Users | Gross volume per chamber |
|---|---|
| 2 | 0.57-0.91 m³ |
| 4 | 1.15-1.83 m³ |
| 6 | 1.72-2.74 m³ |
Each row is users × 0.63-1.0 × 365 ÷ 0.8 ÷ 1,000.
A Third Chamber Needs Less Total Volume
With three chambers, one fills while two rest, so each batch rests for two fill periods. To rest 12 months, each chamber needs a fill time of only 6 months: for four people, 4 × 0.63 × 182.5 ÷ 0.8 = 575 L = 0.57 m³ to 4 × 1.0 × 182.5 ÷ 0.8 = 912 L = 0.91 m³ each, a total of 1.7-2.7 m³ against 2.3-3.7 m³ for two large chambers. More walls and more doors, less volume.
Measure Your Own Rate
The inputs above vary with diet and cover habits. Run a bucket for a week (Simple Bucket System), measure the litres used per person per day by marking the bucket in 1 L steps, and put your own figure into the formula. Also expect the contents to settle: the first-month page notes finished compost is 10-20% of the original volume, so the emptied volume is small.
Common Mistakes
- Using the parent page’s 1 m³ as a fixed rule. It fits two to three people for a year (800 L ÷ 365 days = 2.2 L per day, which is 2.2 to 3.5 people at 0.63-1.0 L each), or four people for 7 to 10 months.
- Forgetting the fill fraction. The last 20% holds the final cap of cover.
- Letting guests and growth outrun the design. Rest time falls in direct proportion to extra users.
Safety
A chamber large enough to hold a year of waste is a confined space
Fermenting waste can use up the oxygen and release hydrogen sulphide, which deadens the sense of smell at dangerous concentrations, so smell is not a safety check, and methane, which is flammable. Never enter a chamber. Empty finished compost with a rake or scoop from the access door, ventilate the door open first, and never test the air with a flame. Two people must be present, one outside at all times.
What’s Next
- Learn next: Ventilation & Odor Control — keep the chambers aerobic
- Combine with: Maintenance Schedules — track fill and rest dates