Greenhouse Yield Calculator UK — Annual Harvest Estimate
Greenhouse Yield Calculator
Pick a NORDIC size and the crops you'd grow — we'll estimate annual harvest in kilograms and supermarket-equivalent value. Based on realistic UK home-growing yields with active venting and reasonable care.
8×6 · 5.2 m²
8×10 · 7.8 m²
8×13 · 10.4 m²
8×16 · 13.0 m²
8×20 · 15.6 m²
Estimated annual harvest
~85 kg
≈ £340 supermarket-equivalent value per year
Crop-by-crop breakdown
Estimates are based on UK averaged growing data with NORDIC's 4 mm twin-wall CrystalLight polycarbonate, screw-fixed glazing, and active venting. Actual results vary by season, exposure, varieties chosen, and care intensity. Supermarket-equivalent uses £4/kg average across mixed crops at typical UK retail.
How the yield calculator works
The calculator uses three inputs: greenhouse size, dominant crop, and grower experience. It allocates 70% of the floor area to actual growing space (the remaining 30% covers paths, staging, and seedling stations), then applies crop-specific yield rates per square metre.
Typical UK home-grower yields per square metre with active venting and reasonable care: tomatoes 8–10 kg/m², cucumbers 8–12 kg/m², peppers 3–4 kg/m², leafy greens 2–3 kg/m² (cut-and-come-again), herbs 1–2 kg/m². The experience multiplier — 0.65× for beginner year-one, 1.0× intermediate, 1.4× experienced — reflects the real-world spread between someone learning from scratch and someone with four years of crop rotation, variety selection, and pest management dialled in.
How realistic are these numbers?
They're conservative middle-of-the-road UK figures. A skilled grower with optimised indeterminate tomato varieties on cordon training can hit 15+ kg/m² — Charles Dowding's no-dig approach regularly exceeds these baselines. A first-season beginner working with garden-centre plug plants and minimal feeding usually lands at 50–65% of intermediate yields.
The supermarket-equivalent value uses £4/kg averaged across mixed crops at typical UK retail. Tomatoes alone retail at £5–8/kg in 2026, peppers £6–8/kg, cucumbers £3–4/kg each (about £4/kg by weight). The blended £4/kg is a defensible mid-point. Organic premiums (£6/kg+) and farmers' market prices push this higher; supermarket basic ranges push it lower.
Payback isn't the whole story
The "payback in X years" figure is useful for framing but doesn't capture the full economics. A NORDIC greenhouse is engineered for a 30+ year service life. The frame holds up because the 45×45 mm Swedish pine from sustainably-managed forests cross-section is twice the timber mass of comparable kits, the joints use 1.2 mm galvanised steel (not aluminium that work-hardens and cracks), and the screw-fixed CrystalLight panels don't blow out in storms the way spring W-clip glazing does.
Across 15 years of typical use, a £1,699 NORDIC-L pays back in vegetables roughly 5× over. Across 30 years (the realistic design life with re-treatment every 2–3 years), 10× over. That doesn't count the seasonal extension — your tomato season runs Mar–Oct instead of June–Sept outdoor, doubling the cropping window — nor the variety access (you can grow heritage varieties unavailable in supermarkets) nor the food security against supply shocks.
What the calculator doesn't show
Three things the numbers can't capture:
- Quality: a tomato picked at full ripeness 30 minutes before eating doesn't compare to a supermarket tomato picked green 14 days earlier and gas-ripened. The £4/kg figure understates the experiential value.
- Seasonal extension: outdoor tomato season in the UK runs roughly mid-July to early September. In a NORDIC, you start harvesting late May (with heated propagation) or early-mid June (cold start), and continue through October. Doubles or triples the productive window for the same plants.
- Year-round growing: leafy greens, herbs, overwintered brassicas, and propagation for spring all extend value into months when nothing grows outdoors. The calculator estimates seasonal yields; year-round growing adds 30–50% more value on top.
Frequently asked questions
Why are tomato yields lower in the calculator than I've seen elsewhere?
Many online sources quote yields per plant rather than per square metre. A well-managed indeterminate tomato plant produces 5–8 kg over the season, but it takes ~0.8 m² of space (with vertical training). 5 kg per 0.8 m² = ~6 kg/m². Our 8–10 kg/m² figure for tomatoes assumes dense planting with strict staking. Beginner first-year tends toward 4–6 kg/m²; experienced growers reach 12 kg/m²+.
Do these yields assume heating?
No — the baseline assumes a cold greenhouse with passive solar gain through the 4 mm CrystalLight polycarbonate (which transmits ~85% light and insulates ~33% better than single glass). With heated propagation (electric mat from January) you can shift season-start forward 4–6 weeks, increasing total yield by 15–25%. Active heating through winter (paraffin or electric) extends growing further but adds operating cost.
How do these compare to allotment / outdoor yields?
Per square metre, a well-managed greenhouse produces 2–3× more than the equivalent outdoor space for heat-loving crops (tomatoes, peppers, cucumbers). For cool-season crops (leafy greens, brassicas, peas), the advantage is smaller — maybe 1.3× — but the season is twice as long. The £4/kg supermarket-equivalent value applies equally; a greenhouse just compresses more harvest into less space.
What's the real ongoing cost?
NORDIC kit cost (£1,499–£1,899) is the main investment. Ongoing: re-treatment every 2–3 years (~£40 of wood preservative, 2 hours of labour), occasional polycarbonate cleaning (£0), and modest electricity for propagation mats (~£15/season). No annual replacement parts. See our full cost breakdown for the 10-year total.
Does the yield change if I grow organically?
Yields are roughly comparable to conventional home growing — UK home gardens are typically lower-input than commercial farms regardless of "organic" label. The supermarket-equivalent value goes UP if you compare to organic retail prices (£5–8/kg). So organic home-growing has higher financial value, just not higher yields.