Dehumidifier Size Guide: How Many Litres Per Day You Need
Quick answer: for water damage drying, work out the room volume in cubic metres and divide by 3 for a typical wet carpet-and-plaster job (divide by 6 for a minor leak, by 2.4 for a saturated room), and that is the litres per day you need from an LGR dehumidifier. For most Australian homes that means a 50L unit for a single room, a 70L unit for an open-plan area, and a 90L unit or several machines for commercial floors and warehouses.
"How many litres per day do I need?" is the first question restorers, builders and property managers ask us, and the answer depends on three things: how much air you are drying, how wet the materials are, and what the machine's rating actually means. This guide covers all three so you can size a dehumidifier properly, whether it is a flooded bedroom, a commercial office or a 1,000 m² warehouse.
What "litres per day" really means
Every dehumidifier is sold on a litres-per-day figure, but that number is only true at the test conditions printed next to it. Our Thorair 50L LGR dehumidifier is rated at 50 L/day at 30°C and 80% relative humidity. The 70L model is rated at 30°C and 90% RH, and the 90L model pulls up to 105 L/day at 26°C and 90% RH. Those are warm, saturated conditions, which is exactly what a freshly flooded room looks like on day one.
As the room dries and the humidity drops, every refrigerant dehumidifier extracts less, because there is less moisture in each cubic metre of air passing over the coil. A unit that removes 50 litres on day one in a steaming wet room may remove well under half that on day three at 22°C and 55% RH. That is normal, and it is why you size for the start of the job, not the end.
The other thing to understand is LGR versus standard refrigerant. LGR (low grain refrigerant) units pre-cool the incoming air so they keep extracting at lower humidity and lower temperatures, which is where a basic refrigerant unit stalls. All three Thorair dehumidifiers are LGR units with Panasonic compressors, so they keep working into the final stage of a dry. When comparing brands, compare LGR to LGR; a cheap 50L "commercial" refrigerant unit is not the same machine.
The sizing formula restorers actually use
The water damage industry sizes dehumidification from the IICRC S500 standard, which gives a cubic-feet-per-pint factor for each "class" of water loss. Converted to metric for LGR dehumidifiers, it comes down to a simple rule:
Litres per day required = room volume (m³) ÷ class factor
- Class 1 (factor 6): a small amount of water, only part of the room affected, low-porosity materials. Think a leaking dishwasher on a tiled floor with a wet skirting board.
- Class 2 (factor 3): the whole floor is wet, water has wicked up walls to around 600 mm, carpet and underlay are saturated. This is the most common residential and office flood.
- Class 3 (factor 2.4): water came from above or the room is saturated floor to ceiling: carpet, walls, insulation and ceiling are all wet.
- Class 4: specialty drying of dense materials such as hardwood, concrete slabs or brick. The formula does not apply; these jobs need LGR or desiccant equipment and longer dry times, and are usually quoted on site.
Worked example: a flooded bedroom
A 4 m × 5 m bedroom with a 2.7 m ceiling is 54 m³. The carpet and underlay are soaked and the plasterboard has wicked water up to the power points, so it is a Class 2 loss: 54 ÷ 3 = 18 L/day. A single 50L LGR unit covers that comfortably, with headroom for extraction falling as the room dries.
Worked example: an open-plan living area
An open-plan kitchen, dining and living area of 60 m² with 2.7 m ceilings is 162 m³. A burst flexi hose has soaked the floating floor and the lounge carpet, so again Class 2: 162 ÷ 3 = 54 L/day. That is right at the edge of a 50L unit, so the correct call is a 70L dehumidifier, or two 50L units if you need to split the airflow. An undersized machine does not just dry slowly; it lets the humidity sit above 60% for days, which is when mould starts.
Sizing by space: rooms, offices and warehouses
If you do not want to run the formula on every job, the table below gives the typical pairing for a Class 2 wet loss. Always round up when the number lands between two sizes.
| Space | Approx. volume | Class 2 requirement | Recommended Thorair unit |
|---|---|---|---|
| Bathroom, laundry or small bedroom | Up to 40 m³ | Up to 14 L/day | 50L LGR |
| Large bedroom, home office, hotel room | 40 – 80 m³ | 14 – 27 L/day | 50L LGR |
| Open-plan living area, small office suite | 80 – 200 m³ | 27 – 67 L/day | 70L LGR, or 2 × 50L |
| Whole house ground floor, retail tenancy | 200 – 300 m³ | 67 – 100 L/day | 90L LGR, or 70L + 50L |
| Commercial floor, gym, small warehouse | 300 – 600 m³ | 100 – 200 L/day | 2 × 90L, or 3 × 70L |
| Warehouse or factory | 600 m³ and up | Calculate: m³ ÷ 3 | Multiple 90L units, placed per zone |
Sizing for warehouses and ongoing humidity control
Warehouses are a different problem. Usually nothing is flooded; stock, cardboard and steel are sweating because the building sits at 70–80% RH through a Queensland or northern NSW summer. With no saturated materials to dry, size closer to the Class 1 factor (m³ ÷ 6) and verify with a hygrometer after 48 hours. A 1,500 m³ warehouse works out to roughly 250 L/day: three 90L units spread across the floor, with continuous drains connected, rather than one machine in a corner. Air movement matters as much as extraction in a large shed, so pair the dehumidifiers with air movers to keep the whole volume turning over.
Dehumidifiers are only half the system
A dehumidifier removes moisture from the air. It cannot pull water out of a saturated carpet unless air movers first evaporate that water into the air. The S500 guideline is roughly one air mover per 5–6 m² of wet floor, plus one for every 3–5 m of wet wall. A dehumidifier with no air movers dries slowly; air movers with no dehumidifier just push humid air into the dry rooms next door.
Balance is the point. On a Class 2 lounge-room job you might run one 70L dehumidifier and six snail or square blowers, with the room closed off so the dehumidifier only treats the air that matters.
Drying targets to work towards
You are finished when the materials are dry, not when the air feels dry. Get the room below 60% RH within the first 24 hours (mould growth accelerates above that), then bring it towards 40–50% RH for the remainder of the dry. Meter the carpet, underlay and plasterboard daily against an unaffected reference area in the same building; when the wet area matches the reference, the job is done. If RH is dropping but material readings are not, you need more airflow, not more dehumidification.
Common sizing mistakes
- Buying on the rated litres alone. A 50L rating at 30°C/80% RH is a different machine from a 50L rating at 35°C/90% RH. Compare test conditions, airflow and whether the unit is LGR.
- Ignoring airflow. The 50L moves 300 m³/h; the 70L moves 300–550 CFM and the 90L moves 600 CFM. On a big open floor, air processed per hour matters as much as the extraction rating.
- Leaving the room open. If the doors and windows are open, you are dehumidifying the suburb. Contain the wet area, and you can size to the actual volume.
Which Thorair dehumidifier should you buy?
If you only own one machine, the 70L is the most flexible choice: it has the capacity for an open-plan flood and the wheels, handle and semi-pneumatic tyres to get up a flight of stairs. If you mostly dry single rooms, strata units or hotel rooms, the 50L is the compact option. If your work is commercial, or you want fewer machines on a big job, start with the 90L and its 600 CFM of airflow. All three ship free Australia-wide except to WA ($50) and TAS ($200), and our team on 1300 263 551 will run the numbers on a specific job before you order. For a broader comparison of models and features, see our best dehumidifier Australia buyers' guide.
Frequently asked questions
How many litres per day do I need for a flooded room?
Measure the room volume in cubic metres and divide by 3 for a typical soaked-carpet flood. A 4 m × 5 m room with a 2.7 m ceiling needs about 18 L/day, so a 50L LGR dehumidifier is the right size with headroom for falling extraction as the room dries.
Is a bigger dehumidifier always better?
No. Oversizing wastes money and power and does not fix poor airflow, the usual reason a job dries slowly. On large floors, two medium units placed apart usually beat one large unit in the middle, and they can be redeployed to two separate jobs next week.
What is the difference between a 50L and a 90L dehumidifier in practice?
Capacity and airflow. The 50L is rated at 50 L/day and 300 m³/h and suits single rooms. The 90L is rated at up to 105 L/day at 26°C/90% RH with 600 CFM airflow, which is what you want for commercial floors, warehouses and multi-room floods. The 70L sits between them and is the best all-rounder.
How long will it take to dry a flooded house?
A properly sized and balanced setup (dehumidifier plus air movers, room contained) typically takes three to five days for a Class 2 residential loss. Class 3 losses and dense materials such as hardwood or slab take longer. Daily moisture-meter readings tell you when it is finished.
Can I use a domestic dehumidifier for water damage?
Domestic units are built for comfort, not drying. They lack the extraction, airflow and continuous drain needed to get a wet room below 60% RH quickly, which is the window in which mould takes hold. For anything beyond a damp cupboard, a commercial LGR unit is the correct tool.
What humidity level should I aim for?
Get below 60% RH within the first day and work towards 40–50% RH for the rest of the dry, then verify the materials with a moisture meter against a dry reference area before you pack up.
Ready to size your next job?
Browse the full range of Thorair commercial LGR dehumidifiers, or call 1300 263 551 with your room dimensions and we will tell you exactly which unit, or which combination, will get the job dry.
