Capacity & Usage Definitions for Composting Toilets

Choosing the right composting toilet is not only about how many people may use it. It is also about how often they use it, the type of composting system being selected, whether the system has a leachate pathway or not, and whether the installation and operating conditions remain within the intended design range.

At WCTNZ®, published user ratings are planning guides used to support correct model selection. They must be read together with actual occupancy, system type, ventilation, climate, consumables use, maintenance, and the practical operating conditions of the site.

At a glance

Full-time use = generally more than 3 days in a week
Part-time use = generally 2 to 3 days in a week
Guest use = generally fortnightly, occasional, weekend, holiday, or short-stay use
Range ratings such as 2–3 or 3–4 should be read conservatively, with the lower number as the standard planning load
Zero-leachate systems should be read more conservatively than leachate-drain systems

How WCTNZ® Defines Use

Full-time use

Full-time use means ongoing residential use as part of normal living occupancy. As a WCTNZ® planning guide, any one user regularly using the toilet for more than 3 days in a week should generally be treated as a full-time user.

Part-time use

Part-time use means non-permanent use. This includes occasional, weekend, holiday, intermittent, or short-stay occupancy. As a WCTNZ® planning guide, 2 to 3 days in a week is generally treated as part-time use, provided the use is not effectively ongoing residential occupancy.

Guest use

Guest use is treated the same as part-time use, but should still be counted where it materially increases the real operating load on the toilet. As a WCTNZ® planning guide, guest use is generally treated as fortnightly, intermittent, occasional, or short-stay occupancy rather than permanent day-to-day living.

Holiday use

Holiday use is generally treated as part-time use. In New Zealand terms, this often means bach, cabin, caravan, or other non-permanent dwelling use. Where holiday use becomes frequent, extended, or close to ongoing residential use, it should be treated more conservatively.

How to Read a Range Rating

Where a toilet is rated as a range, such as 2–3 people or 3–4 people, that does not mean the system is automatically suited to the top number in all full-time situations.

As a WCTNZ® planning standard, the lower number should generally be read as the standard, more conservative full-time planning load. The higher number may be achievable only where occupancy pattern, system type, installation quality, ventilation, consumables use, climate, and overall system management remain favourable.

A range rating is therefore not a blanket entitlement to the top number. It is a working operating range that depends on the real conditions of use.

How WCTNZ® Reads User Load

Use this guide first. It helps interpret occupancy style, system type, and whether the published rating should be read conservatively.

Start with how often the toilet is used.
More than 3 days in a week is generally treated as full-time use. Two to 3 days in a week is generally treated as part-time use. Fortnightly, weekend, holiday, and guest use is generally treated as part-time / guest use.
Then read the published rating carefully.
A rating shown as 2–3 or 3–4 is an operating range, not an automatic entitlement to the top number in all conditions.
Then consider the system type.
Different composting system types do not handle user loading in the same way. Self-contained, split, urine diversion, batching, drying, and hybrid systems should not all be interpreted the same way.
Then check whether the system is leachate or zero-leachate.
Zero-leachate systems should be read more conservatively. Leachate-drain systems may allow broader guest tolerance, but still not unlimited use over rating.
When in doubt, size to the higher real-world load.
Where actual use is mixed, growing, uncertain, close to the next model size, or likely to move from part-time occupancy toward permanent dwelling use, WCTNZ® recommends sizing to the higher real-world load.

Why System Type Matters

Composting toilets do not all handle user loading in the same way. System type has a direct bearing on how conservatively published user ratings should be read.

Self-contained systems

Self-contained systems place the composting chamber inside the pedestal itself. These systems generally have tighter working space and can be more sensitive to moisture balance, loading, and user behaviour than larger sub-floor systems.

Split systems

Split systems connect the pedestal to a composting chamber below floor level. These systems generally allow for larger chambers, better retention time, and lower maintenance intervals, so they may offer more tolerance where user loading is mixed or higher over time.

All-in-one and urine diversion systems

Some systems collect solids and liquids together in a single chamber, while others separate urine from solids. This distinction materially affects moisture management, guest loading tolerance, and how conservatively the published rating should be read.

Continuous, batching, hybrid, and drying systems

Some systems work by batch rotation, some by long retention time, some by hybrid wet-to-dry staging, and some by drying or desiccation. Each of these system types responds differently to moisture, peak usage, and loading spikes, which is why not all user ratings should be interpreted the same way across all technologies.

Leachate vs Zero-Leachate Capacity Interpretation

One of the most important distinctions in reading a user rating is whether the system has a leachate pathway or whether it is a zero-leachate system.

Zero-leachate systems

Zero-leachate systems operate without a soak-away and therefore have tighter moisture margins. Where a zero-leachate system is already being used at its full-time rating, there will not usually be a meaningful guest rating over and above that full-time use.

In practical terms, any guest allowance on a zero-leachate system should generally be read as overnight / occasional only, not as an additional ongoing occupancy allowance.

Where a zero-leachate system is already carrying its full full-time load, it should not generally be assumed to have additional guest capacity beyond that short-stay / overnight context.

Leachate-drain systems

Systems with a leachate drain or soak-away can generally offer a wider tolerance for guest loading than zero-leachate systems, because excess liquid has a managed discharge pathway.

As a WCTNZ® planning guide, guest loading on leachate-based systems may be read in a broader practical range, often up to a 2–4 part-time / guest context, with the lower end treated as the more conservative value.

These are still planning guides only. Actual tolerance for extra users depends on the type of system, ventilation, climate, consumables, and how the toilet is being operated in the real world.

Capacity Interpretation Chart

Situation	How WCTNZ® reads it	Planning outcome
Permanent dwelling / regular ongoing living	Normal residential occupancy	Rate as full-time use
More than 3 days in a week	Regular use beyond intermittent occupancy	Rate as full-time use
2 to 3 days in a week	Intermittent but recurring use	Rate as part-time use
Fortnightly / weekend / holiday / guest use	Non-permanent occupancy	Rate as part-time / guest use
Rating shown as 2–3 or 3–4 users	Operating range, not a flat entitlement to the top number	Lower number = standard planning load; upper number = favourable conditions only
Zero-leachate system already at full-time load	Tighter moisture margin and no discharge pathway	No meaningful extra guest allowance beyond overnight / occasional use
Leachate-drain / leaching system	Better excess-liquid tolerance than zero-leachate systems	Guest range may be broader; read conservatively at 2 before assuming 4
Mixed or uncertain site use	Real occupancy may exceed optimistic rating assumptions	Size to the higher real-world load

This chart is a WCTNZ® planning guide. It is intended to help interpret system sizing and published ratings more accurately across different types of composting toilets.

Why User Count Is Only Part of the Picture

User numbers do not operate in isolation. Real composting toilet performance is also affected by ventilation design, room pressure, climate, moisture balance, consumables type and dosing, maintenance quality, and the type of toilet system being used.

This is why a toilet may sit within a nominal occupancy figure on paper but still perform poorly if the real operating conditions fall outside the intended range.

WCTNZ® Recommendation

Where actual use is mixed, increasing, close to the next model size, or likely to move from part-time occupancy toward permanent dwelling use, WCTNZ® recommends selecting the system to suit the higher real-world load, not the most optimistic interpretation of the rating.

When in doubt, choose the model and the system type that best match the true living pattern of the site.

Related Guidance

To choose the right system, also read the wider WCTNZ® guidance and the individual detail pages for the brand or system type you are considering, including:

Types of Composting Systems
Tiny House Toilets for Sustainable Living in New Zealand
Composting Systems Warranties
Passive Ventilation Design Parameters Guide
Room Ventilation and Extraction Systems Guide
Frequently Asked Questions (FAQs)
How Often are Composting Toilets Emptied?
What are Composting Toilets and how do They Work?
the individual detail, technology, startup, maintenance, and troubleshooting pages for the brand or system type you are considering

Important WCTNZ® Guidance & Disclaimer

Published user ratings should be treated as planning guidance only and not as stand-alone guarantees. All sites, users, and installation conditions contain variable factors. These include climate, ventilation design, room extraction, moisture balance, consumables use, maintenance standard, power supply where relevant, and how closely the chosen system type matches the true occupancy pattern.

WCTNZ® can work with clients before purchase and after purchase to help guide system type, system capacity, operating limits, and the interpretation of published ratings for a particular application. Where there is any uncertainty, WCTNZ® recommends sizing conservatively and discussing the likely real-world use case in advance.

For composting toilets, successful performance remains partly dependent on the operator. The system hardware can only perform within the limits of the installation and the way it is used and managed over time.

For further guidance, also read Types of Composting Systems, Composting Systems Warranties, Passive Ventilation Design Parameters Guide, Room Ventilation and Extraction Systems Guide, Tiny House Toilets for Sustainable Living in New Zealand, and the individual detail, technology, startup, maintenance, and troubleshooting pages for the brand or system type being considered.

These capacity and usage definitions are WCTNZ® planning and specification standards used to support correct model selection, installation guidance, and user expectation setting across composting toilet systems.