How many seeds do I need to sow for 48 seedlings at 85% germination? The usual shortcut — target divided by germination rate — misses two things: sowing two seeds in one cell makes that cell much more likely to produce at least one seedling, and not every seedling survives to transplanting. This page does the per-cell probability properly, then turns it into cells, seeds, trays and packets, including repeated sowings (successions). It runs entirely in your browser and saves or sends nothing.
Per-cell success = 1 − (1 − germination rate)seeds per cell. That is the chance that at least one seed in the cell germinates — if two germinate, you thin to the strongest one, you do not get two seedlings from one cell. Cells to sow per succession = ceil(target seedlings ÷ (per-cell success × survival rate)). Seeds per succession = cells × seeds per cell. Trays per succession = ceil(cells ÷ cells per tray). Packets are shown two ways: per batch, and for all successions bought together, because leftover seeds carry over between batches.
With the defaults above: target 48 seedlings, 85% germination, 2 seeds per cell, 90% survival. Per-cell success = 1 − (1 − 0.85)2 = 1 − 0.0225 = 97.75%. Cells per succession = ceil(48 ÷ (0.9775 × 0.90)) = ceil(54.56) = 55 cells. That is 110 seeds and 1 tray of 72 cells per succession (55 cells used). For 2 successions: 110 cells, 220 seeds, 2 tray-fillings, and 3 packets bought together (ceil(220 ÷ 100)), or 2 packets per batch if bought separately. The naive target ÷ germination rate gives 56.5, which looks close here but ignores both the per-cell effect and survival — with 1 seed per cell and lower survival the gap grows.
Is this a guarantee? No. Germination varies with seed age, storage, temperature, moisture and light, and a packet rate is an average, not a promise for your batch. Treat every result as an estimate and sow a few extra cells for your most important crops. What germination rate should I enter? The packet rate if you have it, otherwise your own test: germinate 20–50 seeds on damp paper and count how many sprout. Do trays get reused across successions? Usually yes, which is why the total is shown as tray-fillings, not trays to buy — check your own stock before buying more. What about direct sowing outdoors? The same arithmetic applies, but outdoor survival is usually lower and more variable, so use a lower survival rate. Is this a buying recommendation? No. It only counts cells, seeds, trays and packets; you check seed variety, price and availability yourself. This page is not professional horticultural advice.
This is a free, single-purpose calculator. All calculation happens in your browser. No account, no upload, no tracking by this page beyond any ads Google may serve after approval.