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What size sewage holding tank do you need? A water-use calculation

A sewage holding tank (Estonian reoveemahuti; in legal texts reovee kogumismahuti; UK readers may know a sealed tank as a cesspool or cesspit) only stores wastewater until a tanker pumps it out. There is no treatment and no outlet, so everything that goes down your drains must be driven away. “What size holding tank do I need?” therefore depends on two things: how much water your household uses and how often you are prepared to pay for a tanker. The short answer: people × litres per day × days between emptyings, plus a reserve, capped by the tankers working in your area. Below are the formula, its assumptions, worked examples and the matching Vesmaco volumes from 2 to 15 m³.

Start from your own water-meter readings

Almost every litre that passes your water meter ends up in the holding tank, so the meter beats any rule of thumb.

  1. Take one month’s consumption from your water bill, or note the meter reading (m³) on two dates about a month apart.
  2. Convert it: litres per day = m³ used × 1,000 ÷ number of days.
  3. If use varies through the year, work with the busiest month, not the annual average.
  4. Subtract water that never reaches the drains (garden watering) only if you can estimate it; otherwise keep it as a margin.

Illustration only: 9.6 m³ in 30 days is 320 L per day; for three people, about 107 L per person per day.

No meter yet — a new build, or a private well without one? Use the official planning range: the Estonian ministry’s guide to on-site wastewater systems (2015) plans for 100–150 L of wastewater per resident per day. Calculate with both ends of the range and switch to real readings as soon as you can — your own meter data takes precedence.

This is also why rules of thumb found online disagree. “So many cubic metres per person per month” is the same arithmetic with hidden assumptions (100–150 L a day equals 3–4.5 m³ per person a month), and “a house needs 10–15 m³” ignores how many people live there and how often the tanker comes.

The formula and its assumptions

required volume (L) = persons × litres per person per day × (target days between emptyings + reserve days)

Check any tank size with the same formula turned round:

days between emptyings = usable volume (L) ÷ (persons × litres per person per day)
emptyings per year     = 365 × occupancy share ÷ days between emptyings

Every input is a visible assumption:

  • Litres per person per day — your meter figure, or the 100–150 L planning range.
  • Target days between emptyings — your choice: a larger tank costs more to buy and bury, a smaller one means more tanker visits. Yearly desludging figures quoted for septic tanks describe sludge removal from a different kind of system and must not be used here.
  • Reserve days — the time between ordering the tanker and its arrival; ask your local emptying company. The ministry guide says a holding tank must never overflow and, for dipstick checks, advises ordering when the level passes about half the diameter. With a level alarm, the reserve is the volume above the sensor; on the wireless alarm offered with Vesmaco tanks, the sensor cable length sets the alarm level.
  • Usable volume — the examples below treat usable volume as equal to nominal volume for simplicity. In reality the alarm level leaves part of the tank as reserve, so the usable volume is somewhat below the nominal volume and real intervals are somewhat shorter. For an exact figure, check the drawing for your model and the alarm height you set.
  • Occupancy share — 1 for a permanent home; for a seasonal one, the fraction of the year in use.
  • Only household wastewater enters the tank. Rainwater, drainage water and groundwater are not in the formula and must be kept out.

One legal minimum to check

In one situation the law sets a floor. Under the Public Water Supply and Sewerage Act (ÜVVKS) § 47, if your property lies in a sewage collection area (reoveekogumisala) of 2000 population equivalents or more and cannot connect because public sewerage is missing, you must install a leak-tight holding tank large enough to hold at least two weeks’ wastewater (§ 47(7)). In return the water company must organise the emptying service (purgimisteenus) at your request, and you are entitled to it twice per calendar month (§ 47(10)). On planning figures, two weeks for four people means 5.6–8.4 m³.

This rule covers that situation only. Elsewhere, look in your municipality’s rules on on-site wastewater treatment and removal (reovee kohtkäitluse ja äraveo eeskiri, Water Act § 104(7)) — check your own municipality before you order.

Worked examples: 2, 4 and 6 people

Assumptions for both tables: permanent occupancy, 100–150 L per person per day, nominal volume, no reserve days added yet.

Volume needed for a chosen interval

Household Daily wastewater Every 7 days Every 14 days Every 28 days
2 people 200–300 L 1.4–2.1 m³ 2.8–4.2 m³ 5.6–8.4 m³
4 people 400–600 L 2.8–4.2 m³ 5.6–8.4 m³ 11.2–16.8 m³
6 people 600–900 L 4.2–6.3 m³ 8.4–12.6 m³ 16.8–25.2 m³

How long a given tank lasts

Tank (nominal) 2 people 4 people 6 people
3 m³ 10–15 days 5–7.5 days 3.3–5 days
5 m³ 17–25 days 8–12.5 days 5.5–8 days
10 m³ 33–50 days 17–25 days 11–17 days
15 m³ 50–75 days 25–37 days 17–25 days

Step by step for a family of four wanting the tanker about every two weeks:

  1. Daily wastewater: 4 × 100–150 L = 400–600 L.
  2. Fourteen days: 5,600–8,400 L.
  3. Reserve, assuming the tanker comes within three days of your call (ask your company): 3 × 400–600 L = 1,200–1,800 L.
  4. Required volume: 6,800–10,200 L. Rounded up to the next volume in the Vesmaco range, that is 7 m³ at 100 L per person and 12 m³ at 150 L. Meter data narrows this: at a measured 110 L per person, 4 × 110 × 17 = 7,480 L, so 8 m³.

The official benchmark fits the formula: the ministry guide gives at least 5 m³ as the optimal size for a four-person family living permanently, emptied about once a week. By the formula, 5 m³ lasts 8–12.5 days at nominal volume — roughly weekly once you order before the tank is full.

The drawback, stated plainly: emptying is the running cost of a holding tank. Four people with 5 m³ need about 29–44 tanker visits a year; with 10 m³, about 15–22. Budget with annual cost = emptyings per year × your local price per visit. Intervals are covered in how often a sewage holding tank should be emptied.

Tanker capacity sets a practical upper limit

A tank that holds more than the tanker cannot be emptied in one visit. The ministry guide (2015) says small tankers (paakauto) carry 4–5 m³ and the largest about 10 m³, and advises checking what is available locally.

Before fixing the size, ask two or three emptying companies serving your address:

  • what load their vehicle takes in one visit;
  • whether they charge per visit or per cubic metre, and what a second load costs;
  • their usual notice time — your reserve days.

Then apply the cap: choose the smaller of your calculated volume and the largest single load available locally, unless you knowingly accept two loads per emptying. Where ÜVVKS § 47 applies, its two-week minimum still comes first. Six people wanting a 28-day interval would need 16.8–25.2 m³ — beyond any single tanker and the 15 m³ top of Vesmaco’s range. If a tanker of about 10 m³ serves your area, the realistic plan is 10 m³ emptied every 11–17 days.

Permanent, seasonal or rental use: how the pattern changes the result

Permanent home. Occupancy share is 1; size for a busy month. Intervals are short, so there is little slack — the ministry guide calls a level sensor advisable.

Summer house or weekend use. Size for the period of use; an annual average would mislead. Four people staying two days a week produce 800–1,200 L a week, so 3 m³ lasts about 2.5–3.75 weeks and 5 m³ about 4–6. The occupancy share gives the yearly count: 40 days of use a year with a 5 m³ tank means 40 ÷ 8–12.5 = 3–5 emptyings. When the smallest models are genuinely enough is covered in 2–3 m³ sewage holding tanks.

Holiday rental or campsite. Size for the peak: guests at full occupancy × litres × nights between tanker visits. Eight guests produce 800–1,200 L a day on planning figures, so 10 m³ lasts 8–12.5 days when fully booked. The planning range was written for residents, so prefer meter readings from your busiest weeks.

From result to model: Vesmaco tanks 2–15 m³

Round your result up to the next volume in the range. Vesmaco manufactures 19 horizontal reinforced-fibreglass (GRP) holding tanks:

Volume Diameter families available
2 m³, 3 m³ D1100
4 m³, 5 m³ D1100 or D1400
6, 7, 8, 9, 10 m³ D1400 or D1600
12 m³ D1600
15 m³ D1600 or D2000

Where a volume comes in two diameters, the larger one gives a shorter tank but a deeper excavation. Dimensions, weights and drawings are in the model table; choosing a diameter for your plot is explained in holding tank dimensions and drawings. If your result falls between the two most common sizes, see 5 m³ or 10 m³: which size should you choose?

Frequently asked questions

How much wastewater does one person produce per day? The ministry’s 2015 guide plans for 100–150 L per resident per day. It is a planning range, not a measurement: your own water-meter readings take precedence.

Is 5 m³ enough for a family of four? As a minimum, yes — if you accept roughly weekly emptying; that is the ministry guide’s benchmark. For a tanker every two weeks the same family needs about 7–12 m³ including a reserve, depending on its measured water use. On planning figures 5 m³ holds less than two weeks’ wastewater for four people, which matters where ÜVVKS § 47(7) applies.

Is a bigger tank always better? No. The yearly volume of wastewater stays the same; a bigger tank only divides it into fewer visits. That helps where the local price is per visit and the tanker can take the whole load. Where the charge is per cubic metre — as under ÜVVKS § 48(2) — tank size does not change the bill. A bigger tank is also longer or deeper to bury.

Send us your calculation. Put your numbers — people, litres per day and their source, the interval you want, reserve days and the largest tanker load available locally — into the quote form or call +372 641 8550, and we will suggest matching Vesmaco models with their drawings. Vesmaco manufactures and delivers the tanks; installation is done by your installer and emptying by your local emptying company.

Sources