Work out the ventilation a dwelling needs under Approved Document F Volume 1 of the Building Regulations. Enter the floor area, bedrooms and wet rooms and the calculator returns the whole dwelling ventilation rate, extract rates for kitchens and bathrooms, purge ventilation requirements and an indicative MVHR unit sizing.

What Part F requires

Approved Document F sets minimum ventilation standards for dwellings in England, and its 2021 edition tightened requirements significantly as homes became more airtight. Compliance rests on four elements, all of which this calculator covers:

Whole dwelling ventilation. The continuous background rate the whole home needs, determined by the number of bedrooms with a floor area check. For a typical dwelling this starts at 19 L/s for one bedroom and rises with each additional bedroom, and the rate must also meet a minimum of 0.3 L/s per square metre of internal floor area. The calculator applies both tests and reports the governing figure.

Extract ventilation. Wet rooms need either intermittent extract fans or continuous mechanical extract at minimum rates: kitchens require the highest rates, with utility rooms, bathrooms and sanitary accommodation each having their own minimums, and the rates differ between intermittent and continuous systems. The calculator lists the required rate for every wet room you enter.

Purge ventilation. The ability to rapidly ventilate a room, normally through openable windows, sized against the room volume. Typically this means openings equivalent to a percentage of floor area depending on how the window opens.

Background ventilators. For naturally ventilated systems, trickle ventilator requirements in habitable rooms and wet rooms.

Sizing an MVHR system from the results

If the dwelling uses mechanical ventilation with heat recovery (System 4 in Part F terms), the whole dwelling rate becomes the continuous supply and extract duty, and the sum of the wet room extract rates sets the boost condition. The calculator reports both, plus a suggested unit duty with headroom, so you can shortlist MVHR units whose certified performance covers the boost rate at a sensible specific fan power rather than running flat out.

A unit selected at or near its maximum will be noisy, inefficient and have no commissioning margin. As a rule of thumb, choose a unit whose boost requirement sits at no more than around 70 percent of its maximum capacity.

Methods and standards

Rates and rules follow Approved Document F Volume 1 (Dwellings), 2021 edition as amended, covering whole dwelling ventilation rates, intermittent and continuous extract minimums, and purge ventilation provisions. The tool has been verified against the tables and worked examples in the Approved Document.

Outputs are indicative and intended for early stage design and product selection. Part F compliance is assessed on the installed system as commissioned, and responsibility for demonstrating compliance rests with the designer and installer. Always confirm the final design against the current edition of the Approved Document and any Building Control requirements for the project.

Who this tool is for

Installers quoting an MVHR or extract system and needing defensible airflow figures for the quote. Self builders and developers checking what a plot needs before speaking to suppliers. Architects confirming that ventilation strategy and window design will satisfy purge requirements at planning stage. Engineers producing a quick compliance baseline before detailed design.

Frequently asked questions

What is the minimum ventilation rate for a house under Part F? The whole dwelling ventilation rate depends on the number of bedrooms, starting at 19 L/s for a one bedroom dwelling and increasing with each additional bedroom, subject to a minimum of 0.3 L/s per square metre of floor area. The higher of the two figures applies. Enter your dwelling details above for the exact rate.

What extract rate does a kitchen need? Under Part F, a kitchen needs an intermittent extract rate of 30 L/s adjacent to the hob or 60 L/s elsewhere in the room, or 13 L/s if extract is continuous. Bathrooms need 15 L/s intermittent or 8 L/s continuous. The calculator lists rates for every wet room in the dwelling.

What is purge ventilation? Purge ventilation is the ability to ventilate a room rapidly, for example to clear cooking smells, fumes from painting or overheating in summer. It is usually provided by openable windows sized relative to the room floor area, with the required opening area depending on the window opening angle.

How do I size an MVHR unit? Size the unit so its continuous rate meets the whole dwelling ventilation rate and its boost rate covers the sum of all wet room extract rates, then add commissioning headroom of around 30 percent. A unit running near its maximum is noisy and inefficient. The calculator reports both duties and a suggested unit size.

Does Part F apply to extensions and refurbishments? Yes, work on existing dwellings must not make ventilation worse, and where windows are replaced or the home is made more airtight, ventilation provisions often need upgrading. New wet rooms created in existing dwellings need extract ventilation to the same rates as new build.

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