Cold frame size inputs
Enter interior dimensions first. Exterior dimensions include wall thickness.
Example data table
| Interior (L x W) | Front / Back height | Wall thickness | Overhang | Suggested lid size | Glazing panels |
|---|---|---|---|---|---|
| 48 in x 24 in | 12 in / 18 in | 0.75 in | 1 in | 50 in x 26–27 in | 1–2 |
| 60 in x 30 in | 14 in / 22 in | 1.00 in | 1.5 in | 63 in x 33–34 in | 2–3 |
| 150 cm x 75 cm | 35 cm / 55 cm | 2.0 cm | 3 cm | 156 cm x 82–84 cm | 2–4 |
Lid width varies with slope and height difference.
Formula used
- Exterior dimensions: Lext = L + 2T, Wext = W + 2T
- Slope length (side): slope = sqrt(W^2 + (Hback - Hfront)^2)
- Lid size: lid_len = L + 2O, lid_wid = slope + 2O
- Interior volume: V = (L x W) x ((Hback + Hfront) / 2)
- Lumber length: (2 x perimeter) + corner posts, then add allowance
- Vent area suggestion: 12% of footprint for passive control
This calculator uses a practical trapezoid-profile approximation.
How to use this calculator
- Choose your unit system and enter interior length and width.
- Set front and back heights to match your sun angle goals.
- Enter wall thickness and an overhang for better sealing.
- Add your glazing panel size so the lid plan stays realistic.
- Click Calculate size and review lid, lumber, and vent results.
- Download CSV or PDF for your cut list and shopping.
For windy sites, increase overhang and add latches.
Sizing for bed efficiency
Cold frames perform best when the interior footprint matches the productive bed area. This calculator uses interior length and width, then adds wall thickness to estimate exterior clearance. That helps avoid frames that crowd paths or irrigation. Choose durable rot-resistant lumber and consider sealing joints to reduce heat loss and extend service life through wet seasons longer outdoors. For tray work, size depth so standard flats sit square with lift space. Consistent sizing across multiple frames simplifies lid sharing and replacement glazing.
Slope geometry and light capture
Lid width comes from the sloped side length: bed width is the run and the back‑to‑front height difference is the rise. Steeper slopes shed rain and snow faster, while gentler slopes can spread light in mild winters. If front height nears back height, slope shortens and moisture lingers. Balance height for winter sun with wind exposure and hinge strength.
Material takeoff with allowance
Lumber length is estimated for base rails, top rails, and corner posts, then multiplied by your allowance percentage. Use allowance for kerf, knots, and upgrades like hinge rails or mid‑span braces on long lids. If you switch to thicker framing, update wall thickness so exterior dimensions remain accurate. Wall area helps plan insulation boards or reflective wraps.
Glazing panels and fit planning
Fixed glazing panels often drive cost, so fit matters. The planner estimates panel count after subtracting edge gaps. For polycarbonate, keep clearance for thermal expansion; for glass, allow weather stripping and safe handling. If panels are pre‑cut, size the frame around them. If framing is fixed, adjust overhang and gap before trimming glazing.
Vent area for temperature stability
Sun can push cold‑frame temperatures far above ambient. The suggested vent area is near twelve percent of footprint for passive control. Add props, sliding vents, or automatic openers to prevent midday spikes. Pair venting with an inner row cover when nights are cold. Log highs and lows for a week, then refine venting for steady growth.
FAQs
1) Should I enter interior or exterior measurements?
Enter interior length and width. The calculator adds wall thickness to estimate exterior size, so you can check clearances around beds, edging, and paths.
2) Why must the back height be higher than the front?
A higher back creates a sloped lid that sheds water and snow and improves winter sun capture. If both heights are equal, the lid plan becomes flat and less weather friendly.
3) How do I choose a good lid overhang?
Use a small overhang to shed rain and help seal gaps. Increase it in windy or very wet locations, but keep it manageable for opening, hinges, and latches.
4) What does the glazing panel count represent?
It is an estimate of how many fixed panels cover the suggested lid size after allowing gaps. Use it to sanity-check panel purchasing, then adjust lid size to match your available panels.
5) Is the lumber length a full cut list?
No. It is a linear material estimate for perimeter rails and corner posts, plus allowance. Add extra for hinges, braces, handles, and any internal shelf or insulation framing you plan.
6) How accurate is the vent area suggestion?
It is a practical starting target. Local sun, wind, and crop tolerance vary, so monitor temperatures and adjust with props, vents, or an automatic opener until daytime peaks stay controlled.