Free cold frame size calculator: enter length, width, soil depth and plant spacing to get growing area, soil volume and how many plants fit inside.
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Enter the cold frame length and width in metres, the soil depth in centimetres, and the spacing between plants in centimetres. The calculator returns the growing area, the soil volume needed to fill it to that depth, and how many plants fit on a regular grid at that spacing. A cold frame is a bottomless box with a clear lid that traps solar heat, so size it to what you can reach and ventilate, not just to the space available.
Growing area is length multiplied by width, and soil volume follows from depth: one square metre at 20 cm deep needs 200 litres of soil, because 1 m2 × 0.2 m is 0.2 cubic metres, or 200 litres. Plant capacity uses a grid: divide length by spacing and width by spacing, round each down, and multiply. Worked example: a 1.8 m by 0.9 m frame at 20 cm deep gives 1.62 m2 of growing area and 324 litres of soil, and at 30 cm spacing it fits 6 plants along the length and 3 across, or 18 plants. A second example: a 1.2 m by 0.6 m frame at 15 cm deep gives 0.72 m2 and 108 litres of soil, and at 20 cm spacing it fits 10 plants, 5 along the length by 2 across. Always round plant counts down: a fractional plant is not a plant.
Small frames of about 2 by 4 feet suit beginners, are easy to move, and grow salad greens for 1 to 2 people, but hold little seed starting capacity. Medium frames of about 3 by 6 feet are the sweet spot for most home gardeners: serious winter greens plus useful spring seed starting, still manageable to build and ventilate. Large frames of 4 by 8 feet or bigger give serious production but need substantial materials, and two medium frames often beat one large one because each lid can be ventilated separately. Keep width at or below about 4 feet (120 cm) so you can reach the middle without stepping inside, and keep the back higher than the front so the lid slopes, sheds rain and snow, and catches low winter sun. This cold frame size guide also covers dimensions for winter gardening, overheating risks, and hardening off steps in the sections below.
Cold frame ventilation is critical, because a closed frame can reach 80 to 100 degrees Fahrenheit on a sunny day even in winter, and that cooks plants. Vent by propping the lid open when inside temperatures pass about 60 to 65 degrees Fahrenheit, and close before evening to hold heat overnight. On very cold nights below 20 degrees Fahrenheit, add blankets or tarps for extra insulation. An automatic vent opener, a wax cylinder device costing about 30 to 60 dollars, opens and closes the lid with temperature and is the best upgrade for busy gardeners. A thermometer at plant height, away from hot glass, tells you when to vent instead of guessing.
Cold frames shine for cold hardy greens: lettuce, spinach, kale, mache, arugula and Asian greens such as mizuna and tatsoi, planted in autumn for harvest through winter. In spring, start seeds 4 to 6 weeks earlier than outdoors, then harden off greenhouse or indoor transplants by leaving them in the frame with the lid closed for 2 to 3 days, then opening for increasing periods over 7 to 10 days. In summer the same frame propagates cuttings or shelters shade loving plants with shade cloth on the lid. A south facing site with at least 6 hours of direct sun, shelter from wind, level ground with good drainage, and water nearby gives the best results in every season.
Size the frame to the lid before cutting boards: if you already have an old window or a polycarbonate panel, measure it first and build the frame to fit snugly underneath. Glazing options include old windows (best value, pre framed), twin wall polycarbonate panels (lightweight, nearly unbreakable, 10 plus year life), greenhouse plastic (cheapest, 1 to 2 year life), and glass panes (best light, heavy and breakable). Standard heights are about 12 to 18 inches at the back and 8 to 12 inches at the front. Budget builds from scrap lumber run about 15 to 45 dollars plus glazing, standard builds about 60 to 80 dollars plus glazing, and commercial frames cost 200 to 500 dollars.
A 3 by 6 foot frame suits most gardeners. Keep width at or below 4 feet so you can reach the middle, and use the calculator to check growing area, soil volume and plant capacity for your exact size.
Prop the lid open when inside temperatures exceed about 60 to 65 degrees Fahrenheit, and close before evening. An automatic vent opener does this by itself using a wax cylinder that expands with heat.
Yes, easily. A closed frame can hit 80 to 100 degrees Fahrenheit on a sunny winter day. Vent on sunny days even when it is cold outside, and use a thermometer at plant height.
Cold hardy greens such as lettuce, spinach, kale, mache and arugula in winter, plus spring seedlings and hardened off transplants. Warm crops such as tomatoes and peppers do not belong in a cold frame.
Yes, in the Northern Hemisphere a south facing frame with a sloped lid catches the most winter sun and sheds rain and snow.
Old windows are the best value, twin wall polycarbonate is lightweight and durable, greenhouse plastic is cheapest but short lived, and glass gives the best light but is heavy and breakable.
Multiply growing area by depth: 1 square metre at 20 cm deep needs 200 litres. The calculator gives the exact figure for your length, width and depth.
Divide length and width by plant spacing, round each down, and multiply. A 1.8 m by 0.9 m frame at 30 cm spacing fits 18 plants.