Window box buying guide: length, depth and fixing
Updated: October 2026
You have a tape measure on a British sill and several plastic troughs that all look long enough in the photo. A window box buying guide settles length, depth, drainage and fixing before a name is worth comparing. This page, from the Products Compared editorial team, uses amazon.co.uk listings checked on 15 October 2026 and public UK sources. We did not run a lab load test. A qualifying purchase through our links can earn a commission at no extra cost to you. The kept listings sit on the [window box comparison](/hub/door-and-window-security/window-box).
You have a tape measure on a British sill and several plastic troughs that all look long enough in the photo. A window box buying guide settles length, depth, drainage and fixing before a name is worth comparing.
You have a tape measure on a British sill and several plastic troughs that all look long enough in the photo. A window box buying guide settles length, depth, drainage and fixing before a name is worth comparing. This page, from the Products Compared editorial team, uses amazon.co.uk listings checked on 15 October 2026 and public UK sources. We did not run a lab load test. A qualifying purchase through our links can earn a commission at no extra cost to you. The kept listings sit on the window box comparison.
Our top picks
| Badge | Product | Size | Drainage | Mounting | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Bamworld Bamworld 7-pack red window boxes | 43 cm bullet, or 10 mm | Yes, two holes | Floor standing | 7.3 | about £16.14 | View offer |
 ARTKETTY ARTKETTY 30 cm ceramic trough | 30.5 × 8.9 × 6.9 cm | Yes, one hole | Window mount | 8.3 | about £18.99 | View offer | |
| Alternative |  Whitefurze Whitefurze 2 × 60 cm Venetian planters | 60 × 22 × 17 cm | Yes ticked; holes not described | Window mount | 6.9 | about £8.99 | View offer |
On this page
Measure the sill, the swing, and the soil depth
Measure the flat length and the depth from the frame to the front lip before you trust a photo. Primrose’s rule of thumb is a box about as wide as the window and roughly 20 to 25 per cent of its height. That is not a law. A trough in the window swing will stop the opening.
Reject a listing that prints two sizes. B2C is a 38 cm title and also 58 cm in the table. Bamworld’s seven-pack lists 43 cm and 10 × 10 × 10 mm. ZOENHOU says 43 cm and a 43 cm cube. Who each listing suits is on the best window box page.
Drainage you can see
You need an outlet you can see, not only a tick. The Royal Horticultural Society treats drainage holes as essential. Open holes let water leave. Stoppers left in keep the base sealed. A reservoir still needs an overflow, or a wet week leaves the compost sodden.
Whitefurze’s 2×60 cm box says drainage is yes, yet the bullets do not describe holes. The components line says mounting hardware, and one UK review still bought brackets separately. B2C has two rubber stoppers and marked spots. If you cannot see a hole or a stopper you could pull, treat the claim as unverified. Wick boxes are on self-watering or drainage.
Stiffness, material, and wet weight
Wet compost, water and plants are the load, not the empty plastic. There is no laboratory kilogram rating for these boxes. On 15 October 2026 this set ran from about £8.90 to about £26.20. Lechuza’s BALCONERA Color 80, about £46.91, was outside the nine.
The Whitefurze 90 cm trough weighs 700 g empty and UK reviews often say it bows once full. Wood-effect plastic on the B2C box is moulded grain, not timber. See plastic trough or wooden window box.
Fixing without the wrong hole
Do not drill a uPVC sill. Garden News puts brackets into the brickwork under the window, about every 45 cm. That spacing is a rule of thumb, not a British Standard. If the plastic sill is the only solid surface, keep the trough freestanding.
A renter needs written landlord agreement before screws go into the wall. On a listed facade, ask the conservation officer before you fix into the listed fabric. Call a professional for masonry above head height. Do not block an escape opening. Northern Ireland follows the EU general product safety regulation from 13 December 2024. A window box is a planter, not a lock.
Common questions
How long and how deep should a window box be for this sill?
Measure the flat sill, including the swing of the opening window. About 15 cm of soil is a grower’s rule of thumb for herbs and bedding, and about 25 cm is a better start for vegetables. The Garden Centre Association also suggests at least 15 cm wide and 20 cm deep, with holes about 2 cm across. Do not average two lengths printed on one listing.
How do you tell open holes, stoppers and a reservoir apart?
Open holes let water leave as soon as you water. A rubber stopper left in is not drainage. A reservoir still needs an overflow, or the compost stays wet. UK reviews of the Whitefurze 60 cm pair often found pin-pricks or corner screw holes rather than open holes. If you cannot see the outlet, treat the claim as unverified.
Can you drill a uPVC sill, and where do brackets go?
Do not drill a uPVC sill. If the only solid surface is the plastic frame, keep the box freestanding or leave the sill bare. Above head height, ask a professional rather than leaning out with a drill. A gap off the wall, in the Garden News method, is there to reduce damp.
What must be agreed in a rented home or on a listed facade?
A verbal assurance is not the written landlord agreement Shelter England expects before screws go into a rented wall. On a listed facade, ask the conservation officer before any hole in the listed fabric. A freestanding trough that does not mark the frame is a different case, though the lease may still limit what sits outside.
When does a box block an escape window?
Approved Document B, volume 1, for dwellings, sets a clear opening of at least 0.33 m² and at least 450 mm high and wide, with the sill no higher than 1,100 mm. The window must stay open without being held. A trough that stops that opening blocks the escape. If it is the only ledge, do not fit the box.
Why is wet compost the load on an upper sill?
Empty plastic understates the load. Compost, water and plants supply the real weight, and a full saucer adds more. The Royal Horticultural Society says to empty saucers and remove them before winter. If a balcony states a load you cannot interpret, ask the building manager first. There is no lab kilogram rating for these boxes.
Related pages
Sources
Public UK sources and the amazon.co.uk listings in this set. Last checked 15 October 2026.
- RHS: growing plants in containers (retrieved 15 October 2026)
- RHS: how to plant a container (retrieved 15 October 2026)
- RHS: peat-free choices (retrieved 15 October 2026)
- BBC Gardeners' World: self-watering planters (retrieved 15 October 2026)
- Consumer Rights Act 2015, section 9 (retrieved 15 October 2026)
- GOV.UK: General Product Safety Regulations 2005 (Great Britain) (retrieved 15 October 2026)
- Approved Document B, volume 1, dwellings (retrieved 15 October 2026)
- Garden News: how to plant a window box (retrieved 15 October 2026)