RMBOA Blackwater Holding Tank Calculator

RMBOA Black Water Capacity Calculator

This calculator is provided by the River Murray Boat Owners Association (RMBOA) as a public service to all river users.

This calculator allow you to enter information about your black water (sewage) holding tank on your vessel and determine how long you can use your vessel without visiting a pump-out station. These estimates are a guide only and should not be used to design a black water system.

Please note: This version of the calculator allows you include the diversion of galley (kitchen sink) water into the black water holding tank. This may be a valid option for vessels classified as low risk by the EPA. Information about these requirements can be found in the EPA publications.
Members can find additional information about filters and strainers in back copies of The Rambler magazine.

If you have two toilets and two holding tanks, it is likely that the galley water will be diverted into one of those tanks. It is recommended that two calculations are done to determine the expected time between pump-outs. For example, My Lady has two toilets (inside and outside) and two holding tanks. The galley water will be diverted into the black water tank for the external toilet which is used less than the internal toilet - who wants to go outside to the toilet when it is freezing in the middle of a winter's night? As a result, one calculation is made with a lower use of the external toilet, but with the galley water included; and another calculation for the internal toilet with no galley water included. The time between pump-outs is the lowest result that is calculated.

Rectangular Holding Tank

Width:  cm   Height:  cm


Length:  cm

Holding Tank Volume

Tank Volume:   litres

Rectangular Cylinder

Cylindrical Holding Tank

Diameter:  cm

For rectangular tanks, measure the length, width and depth. For cylindrical tanks, measure the diameter of the pipe and its length. The units for each measurement is in cm. When you entered the measurements, select Rectangular or Cylinder before pressing Calculate Volume. If you boat uses multiple holding tanks of the same diameter or width and height, do separate calculations taking into account any greywater diversions and differences in average use. The result in in litres. If you know the volume of your tank in litres, then simply type in the value and move to the step below.


Average Water Used for washing Up

Volume of water  litres

Times Per Day

Times per Day washing up done

The EPA has indicated that low risk vessels that have enough room for containment will be required to contain galley water (primarily washing up water from Kitchen Sink). The easiest way to do this is to divert the sink to the black water holding tank.

This, however, will reduce the number of days that the tank can be used before it needs to be emptied. As this calculator shows, the galley water could be equivalent to another person (or more) onboard using the toilet, so it can have a significant impact on the times between pump-outs. If practical, it may be advantageous to only wash up once per day.

As the requirement is only to contain water that has been used for washing up, consideration should be given to running water into a basin until the water gets hot or using a diverter that dumps the tap water until it is hot. This untainted water can be returned to the river or put on the river bank without causing pollution. This calculator will help you consider these trade-offs.


Toilet Flush Volume

 Flush Volume: litres

Flushes Per Person Per Day

 Flushes: per person per day

Empty at Percentage Full

 Empty at : % Full

The volume of the flush of a caravan style toilet is about 1 litre. Single flush toilets can use up to 12 litres, while the more modern flush toilets range from about three (3) litres up to about 9 litres. The number of times a person goes to the toilet will determine the amount of times the toilet is flushed and hence the total amount of being flushed into the tank. It is not a good practice to fill your holding tank to capacity. It is best to have reserve capacity, rather than being caught days from a pump-out station. It is suggested that planning to empty your tanks at 50~75% capacity will give you that margin and may reduce the amount of chemicals needed to treat your tanks to control odours.


Number of People on Vessel

Number of People:

Time Between Pump-Outs

Days (approx)

If you place pet waste into the toilet, add a fraction of a person to the number of people for a more accurate calculation.

Once you have calculated the capacity of your black water (sewage) holding tank, the RMBOA Sewage Pump-Out Record sheet can help keep track of the stat of your holding tanks. If you record when and where your pump-out, it also provides a record that you dispose of your waste correctly.

Script Last Updated: 12 June 2010 Version: 1.1 Author: Mark Little