The bulb planting calculator above works out how many flower bulbs a bed needs at a spacing you choose, in either a square grid or a staggered triangular layout, and reports the planting depth published by a university extension service for the bulb type selected. It gives the bed area, the resulting density per square foot or square metre, and the count you would need if you switched to the other pattern.
Arb Digital publishes this as ordinary garden arithmetic. It is a planning tool for ordering the right quantity of bulbs, not a horticultural instruction: every depth figure it shows is quoted from the extension source linked below, and the bulbs, the soil and the climate in your own garden are the things that actually decide the outcome.
What This Bulb Planting Calculator Does
Enter the bed dimensions and the spacing you want between bulbs, and the tool converts the area into a bulb count. It handles the two layouts gardeners actually use. A square grid places bulbs on a simple rows-and-columns lattice; a triangular or offset layout staggers each row by half a spacing so every bulb sits equidistant from six neighbours, which fits noticeably more bulbs into the same ground at the same centre-to-centre distance.
The depth figure is not calculated. It is read from a small table of values published by extension services, so it changes when you change the bulb type and is shown as the range the source gives rather than as a single number. For bulbs not in that list, the calculator shows the general rule of thumb instead, which is expressed as a multiple of the bulb rather than as a fixed measurement.
The boundary with our other garden tools is worth stating. This page counts individual plants across an area at a fixed spacing. If you want soil volume to fill the bed, use the soil volume calculator; for a broadcast rate across a lawn rather than discrete planting positions, the grass seed calculator is the right tool.
How to Use It
- Choose your units first, because the spacing box switches between inches and centimetres with it.
- Enter the bed length and width. Split an irregular bed into rectangles and run each separately.
- Pick the bulb type to load its published spacing, then adjust the spacing if you want a denser or sparser display.
- Choose the pattern. Square is easier to lay out; triangular gives a fuller look and a higher count.
- Read the depth alongside the count and check it against the supplier's own guidance for the specific cultivar you bought.
The Formula and How It's Calculated
The count comes from an area divided by the ground each bulb occupies. On a square grid, a bulb at spacing s occupies s² of ground, so:
Bulbs = bed area ÷ s²
In a triangular layout the rows sit closer together — the row offset is s × sin 60°, about 0.866s — so each bulb occupies only 0.866 × s². Dividing by that gives about 1.155 times as many bulbs for the same spacing:
Bulbs = bed area ÷ (0.866 × s²)
Work the default. A bed 10 ft by 4 ft is 40 square feet. A 5 inch spacing is 0.4167 ft, so s² is 0.1736 square feet and the density is 5.76 bulbs per square foot. Multiply by 40 and you need 231 bulbs on a square grid, rounding up. The same bed in a triangular layout takes 5.76 × 1.155 = 6.65 per square foot, or 267 bulbs.
What the Extension Services Say About Depth
Planting depth guidance comes in two forms, and they do not always agree, which is why this page shows the source's own words rather than blending them. Illinois Extension states the rule of thumb for spring bulbs as planting two to three times as deep as the bulb is tall, and is explicit that the depth is measured from the bottom of the bulb, not the top.
Iowa State University Extension gives absolute figures for the common spring bulbs, saying that tulips and daffodils should be planted 6 to 8 inches deep, with crocus and grape hyacinth at 3 to 4 inches, and describes the underlying rule as three to four times the bulb's maximum diameter. Both sources note that the rule applies to spring-flowering bulbs and not to summer bulbs, which have their own requirements entirely.
Why the Pattern Changes the Count So Much
The 15 per cent difference between a square grid and a staggered layout surprises people, because both keep neighbouring bulbs the same distance apart. The reason is that a square lattice wastes space diagonally: each bulb's four nearest neighbours are at distance s, but the diagonal neighbours are 1.41 times further away, and that gap does nothing for the display.
A triangular lattice is the densest possible arrangement of equal circles in a plane. Every bulb has six neighbours all at the same distance, and the empty diagonal disappears. In practice most gardeners plant in loose informal drifts rather than either lattice, in which case the triangular figure is the closer estimate, because a natural scatter packs more like a hexagon than a chessboard.
There is a second reason the staggered figure is usually the better estimate. A square grid only achieves its theoretical count if the bed is an exact multiple of the spacing in both directions, and real beds never are. Every edge leaves a partial row that either wastes ground or forces an uneven margin. A staggered planting absorbs those edges far more gracefully, which is one reason it is the layout most planting plans assume.
Spacing Is a Design Decision, Not a Rule
The spacing published for a bulb is a starting point that assumes you want the plant to have room to build a reserve for next year. Tighten it and you get a denser display in year one at the cost of more competition afterwards; loosen it and the bed looks thin in the first spring but the clumps fill in. Neither is wrong, and the calculator lets you set whatever spacing your design calls for.
One consequence worth planning for: bulbs planted very close together are harder to lift and divide later, and species that naturalise will close the gaps on their own within a few seasons. If a bed is meant to be permanent, the wider spacing usually looks better by year three, which is the opposite of how it looks in the catalogue photograph taken in year one.
Mixed plantings need a little care with the arithmetic. If you are layering two bulb types in the same bed — a common approach, since different species come up at different times — run the calculator once per species at that species' own spacing, and treat the results as two independent counts rather than dividing one total between them. The layers occupy different depths, so they are not competing for the same ground in the way a single-species grid does.
What the Bulb Count Cannot Tell You
An area calculation assumes the whole bed is plantable, and beds rarely are. Established shrub roots, irrigation lines, paving edges and the crowns of existing perennials all remove usable ground, and an unadjusted count will overstate what fits. Measure the plantable area rather than the outline of the bed.
The calculator also says nothing about whether the bulbs will thrive, which depends on drainage, light, soil type, hardiness zone, rodent pressure and the quality of the bulbs themselves. Those questions belong with your local extension service or a knowledgeable nursery, both of which know your conditions in a way that no formula does. Use the count to place your order and the depth as a check against your supplier's instructions.
Arb Digital publishes hundreds of free, no-signup tools covering gardening, science, finance, and everyday maths — no accounts, no stored data.
Browse All Free Tools Contact Arb DigitalCommon Mistakes to Avoid
- Measuring depth from the top of the bulb. Extension guidance measures from the base, which can be several centimetres deeper than you think.
- Using the bed outline as the plantable area when roots, edging and existing plants take up a share of it.
- Ordering the exact count with no allowance for soft or damaged bulbs.
- Applying a spring bulb rule to summer bulbs, which have different depth requirements entirely.
- Assuming a triangular count can be planted on a square grid. Pick the pattern before you order, not after.
Related Free Tools From Arb Digital
For the rest of the bed preparation, the soil volume calculator works out fill for a raised bed, the mulch calculator covers the top dressing, and the compost calculator and fertilizer calculator handle amendments. For measuring the bed itself there is the square footage calculator and the area converter. On the field-crop side, see the crop yield calculator. Browse the full free online tools hub for more.
Frequently Asked Questions
Divide one by the square of the spacing in feet. At 5 inch spacing that is 0.4167 feet squared, or 0.1736 square feet per bulb, giving 5.76 bulbs per square foot on a square grid and about 6.65 in a staggered layout.
Iowa State University Extension states that tulips and daffodils should be planted 6 to 8 inches deep, with crocus and grape hyacinth at 3 to 4 inches. Depth is measured from the bottom of the bulb.
Illinois Extension gives the rule of thumb as two to three times as deep as the bulb is tall, while Iowa State expresses it as three to four times the bulb's maximum diameter. Both note that the rule applies to spring-flowering bulbs rather than summer bulbs.
Because a triangular lattice packs more tightly. Rows sit 0.866 of a spacing apart instead of a full spacing, so each bulb occupies less ground and about 15 per cent more fit into the same area at the same centre-to-centre distance.
The published spacing assumes the plant needs room to build a reserve for the following year. Tighter spacing gives a fuller first season with more competition afterwards. The calculator accepts whatever spacing your design calls for.
Break it into rectangles, calculate each one separately, and add the counts. Measure the plantable ground rather than the outline, since roots, edging and existing plants remove usable space.
The count arithmetic works for any spacing, but the depth figures shown are for spring-flowering bulbs only. Extension guidance is explicit that the spring rule of thumb does not carry over to summer bulbs, so check the supplier's instructions for those.
This calculator performs area and spacing arithmetic and reports planting depths quoted from the university extension sources linked above. It is not horticultural advice for your garden. Soil, drainage, climate and cultivar all affect results, and your local extension service or nursery is the authority on growing conditions where you are.
