A carbon filter is the bit of kit that keeps your grow your own business b and the one beginners most often size wrong. Pick one that's too small and it chokes your fan; pick one that's too big and you've paid for capacity you'll never use. The trick is matching the filter to your extraction fan and your tent volume. Here's how to do that properly, and the sizes we stock.
The quick answer
- 100mm (4") - small tents, 60C60cm to 80C80cm, paired with a 100mm inline fan
- 125mm (5") - 80C80cm to 100C100cm, the common mid-size step-up
- 150mm (6") - 100C100cm to 120C120cm, the size most home grows settle on
- 200mm (8") - large or multi-tent rooms with a 200mm fan to match
If you want one rule: the filter flange should match your fan's size, and the filter's airflow rating (mB3/h) should comfortably exceed your fan's. Same diameter in, same diameter out.
What size carbon filter do I need?
Size the filter to two things - your fan and your tent volume. Work out your tent volume in cubic metres (width x depth x height), then aim to exchange that air every minute or two. A 100x100x200cm tent is 2mB3, so you want a fan moving comfortably more than 120mB3/h once you account for resistance. Your filter then needs an airflow rating that exceeds the fan's, so it never becomes the bottleneck.
As a rough field guide: a 100mm filter suits a small 60 - 80cm tent, a 150mm filter pairs with a 100x100cm to 120x120cm, and a 200mm is for the big rooms. We sort our carbon filters by flange size so you can match them straight to your fan.
How to match a carbon filter to your extraction fan
The filter and fan are a pair - buy them together and the maths gets easy. Match the flange diameter first (a 150mm fan wants a 150mm filter), then check the numbers: a carbon filter adds resistance, so the rule of thumb is to size your fan with 20 - 30% extra airflow to pull cleanly through it. Undersize the fan and the air sits in the tent; oversize the filter relative to the fan and you've simply spent more for no gain.
If you're buying the whole extraction side from scratch, grab the inline fan, filter and ducting as a set so the diameters line up first time.
Where to mount it, and why high matters
Mount the filter at the top of the tent, where the warm, odour-rich air collects. A filter sitting low scrubs the cleanest air in the tent while the smelly air leaks out the ports - all the cost, none of the benefit. Hung high, paired with a fan pulling through it, it scrubs the air before it ever leaves the tent. A 150mm filter is a fair lump of weight once it's loaded with carbon, so hang it on proper rope ratchets rather than trusting the tent bar alone.
How long does a carbon filter last in Irish conditions?
Roughly 12 - 18 months in normal use - but this is Ireland, and our damp air shortens that. Activated carbon absorbs moisture as well as odour, so a tent regularly running above 70% humidity can wear a filter out in 8 - 10 months. Treat replacements as a budget line, not a one-off: plan to swap it on a schedule rather than waiting until you can smell the grow again, because by the time you notice, your neighbours noticed a fortnight ago.
Pop in and we'll size it with you
If you're not sure which filter pairs with your fan, measure your tent and tell us your fan size - we match these up every day at the shop. DIG is Ireland's largest grow shop, based in Dublin, with collection available in store and delivery across Ireland.
Want the full breakdown of how carbon scrubbing actually works and how to time your replacements? Our friends at Grow Good Bud have the free in-depth guide.
FAQ
What size carbon filter do I need?
Match it to your fan and tent. Roughly a 100mm filter for a 60 - 80cm tent, a 150mm for a 100 x 100cm to 120 x 120cm, and a 200mm for larger rooms. The filter's airflow rating should exceed the fan's so it never chokes it.
How do I match a carbon filter to my extraction fan?
Match the flange diameter first - a 150mm fan wants a 150mm filter. Then size the fan with 20 - 30% extra airflow, because the filter adds resistance the fan has to pull through.
How long does a carbon filter last?
About 12 - 18 months in normal conditions, but as little as 8 - 10 months in damp, high-humidity Irish setups, since the carbon absorbs moisture too. Replace on a schedule, not when the smell returns.
Do ONA gel or air fresheners replace a carbon filter?
No! They mask odour rather than remove it. A carbon filter on the extraction is the real solution; scent products are only a stopgap for smell leaking into shared areas.
Where can I buy a carbon filter in Ireland?
DIG (Dublin Indoor Gardening) stocks carbon filters and matching inline fans in all common sizes, with collection in Dublin and delivery across Ireland.
