Choosing the right window box size is one of the most important decisions you'll make before planting up — get it wrong and you'll either have a box that overhangs awkwardly or one that looks lost on a wide sill. Here's how to get it right first time.
Step 1: Measure Your Sill or Fixing Surface
Start by measuring the width of your window sill or the wall space where you plan to mount the box. As a rule of thumb, your window box should be no wider than the sill itself — ideally 2–5 cm shorter on each side to allow for bracket clearance and airflow.
For recessed sills, measure the internal width. For wall-mounted boxes, you have more flexibility, but keep proportions in mind: a box that's too narrow will look sparse, while one that's too wide can feel heavy.
Step 2: Consider Depth and Volume
Depth matters as much as length. Most flowering plants and herbs need at least 15–20 cm of soil depth to establish healthy roots. If you're growing trailing plants like lobelia or ivy, a shallower box (12–15 cm) can work, but for structural plants such as lavender or dwarf grasses, go deeper.
Our metal window boxes are available in a range of depths — check the product dimensions carefully and consider what you plan to grow before ordering.
Step 3: Think About Weight
Metal planters are robust and long-lasting, but a fully planted and watered window box can be surprisingly heavy. Make sure your sill, brackets, or fixings are rated for the load. As a guide, a 60 cm box filled with compost and plants can weigh 8–12 kg when wet.
If you're mounting to a rendered or brick wall, use appropriate rawlbolts and check the wall's condition first. We always recommend consulting a builder if you're unsure.
Step 4: Match the Style to Your Home
Our handmade steel window boxes are finished in a range of colours to complement both traditional and contemporary properties. A longer box suits wide Georgian sashes; a pair of shorter boxes works beautifully on narrower cottage windows.
If you're unsure which size to order, feel free to get in touch — we're happy to advise based on your measurements and photos.
Ready to find your perfect fit? Browse our full range of metal window boxes or explore our garden planters for freestanding options.