Bulb Spacing Calculator

How to use the bulb spacing calculator

1. Measure the bed in feet: length along the rows, width across them. prefill →
2. Pick spacing from the bulb type: 4–6 in for tulips, daffodils and other large bulbs; 2–3 in for crocus, grape hyacinth and other small bulbs. prefill →
3. Enter the bulb's height (tip to base plate) to get planting depth at the four-times rule; leave 0 if you already know your depth.
4. Put the package count in to see how many bags to buy. Bulb bags rarely match bed math, so expect a spare bag or leftovers.
5. Depth guidance assumes reasonable drainage: in heavy clay or very wet sites, plant to the shallower end of the range so bulbs are not sitting in water.

Bulb Spacing: the formula & its source

Spacing is on-center, so every bulb claims a square whose side equals the spacing. Convert feet to inches first:

bulbs per row = length (in) ÷ spacing
rows = width (in) ÷ spacing
total = bulbs per row × rows

Or go straight to density: a square of side s inches holds 144 ÷ s² bulbs per square foot:

bulbs/ft² = 144 ÷ spacing²

Planting depth comes from the bulb's own height:

depth ≈ 4 × bulb height

Large bulbs land roughly 6–8 in deep under this rule, small bulbs 1–3 in.

Bulb Spacing: worked example

Worked example: 10 ft × 4 ft bed, 6 in spacing, 2 in bulbs, 50-count bags. Along 10 ft (120 in): 120 ÷ 6 = 20 bulbs per row. Across 4 ft (48 in): 48 ÷ 6 = 8 rows. Total = 20 × 8 = 160 bulbs, matching the density check, 144 ÷ 6² = 4 bulbs/ft² × 40 ft² = 160. Depth = 4 × 2 = 8 in. Packages: 160 ÷ 50 rounded up = 4 bags.

Example B: small bulbs, loose counts

Example B: a crocus drift with no bag counts. A 6 × 3 ft patch at 3 in spacing holds 72 ÷ 3 = 24 bulbs per row and 36 ÷ 3 = 12 rows, so 288 bulbs, which the density check confirms: 144 ÷ 3² = 16 per ft² across 18 ft². A 1 in bulb plants 4 × 1 = 4 in deep under the four-times rule. Leave the package count at zero when bulbs come loose by the handful, and note that counts this tight drift from a pure area calculation by a few bulbs. Order a small spare rather than running the grid again to the last plant.

Spacing is a grid, not a guess

On-center spacing means every bulb claims a square whose side equals the spacing you chose, so the arithmetic is division in two directions. Convert the bed to inches, divide the length by the spacing for bulbs per row, divide the width for the number of rows, and multiply. The same answer arrives by density instead: a square of side s inches holds 144 ÷ s² bulbs per square foot, which is why 6 in spacing gives four per square foot, 4 in gives nine and 3 in gives sixteen. Both routes should agree, and when they do not, one of the inputs was typed in the wrong unit. Grid rows look formal and are easier to mark out, while scatter-planting drops bulbs where they fall for a natural drift. At identical density, only the pattern changes. Whichever you pick, work outward from one fixed corner so the rows stay parallel, and count as you go: a grid that drifts by half a spacing across the bed is obvious from the path. A quick check against the density figure catches a row that has crept as it ran.

Depth follows the bulb, not the calendar

The four-times rule ties planting depth to the bulb itself: measure from tip to basal plate and multiply by four, which lands a typical 2 in bulb at 8 in and puts small bulbs roughly 1 to 3 in down while large ones sit near 6 to 8 in. Depth decides the following spring as much as spacing does. Plant shallow and bulbs heave during freeze-thaw cycles and bake through summer; plant deep in a cold zone and the first year goes into building energy instead of flowers. Set the base of each bulb on firm soil before backfilling, since a bulb resting in loose fill wobbles and roots poorly. The rule assumes reasonable drainage: in heavy clay or a wet corner, plant toward the shallow end so bulbs are not sitting in water. Firm the soil lightly after backfilling rather than tamping hard, and water once to settle the fill around the bulb without washing the depth away.

Ordering against bags that never fit

Bulb bags are commercial objects and bed maths is pure geometry, so the two rarely divide evenly. Round the package count up, expect leftovers, and keep a spare bag on hand because curves, edging and odd corners nibble a few bulbs that the rectangle never counted. Grade markings add another wrinkle: a bulb sold as 12/14 is a size range rather than a single size, and depth guidance has to be read against the largest dimension you actually receive. If the count comes out at 160 and the bags hold 50, four bags buy 200, a margin of 25 bulbs that disappears into corners, splits and the handful that fail to sprout rather than sitting in the shed forever. Leftovers keep if they are stored cool and dry in paper rather than plastic, and they give you replacements in spring for the spots where nothing came up.

How the bed will read

The total is a purchasing figure first, but the density term is what decides how the bed looks from ten feet away. Four bulbs per square foot looks generous in the package and perfect in the ground, because leaves fan out well past the bulb and close the space once growth starts; tight spacings read as a fuller drift, wider ones as scattered accents. Decide the impression first and let the count follow, rather than buying what fits the budget and spacing whatever is left across the bed. A sparse grid planted to a strict pattern reads as unfinished, while the same bulbs scattered along a curve look deliberate. Whatever pattern you choose, mark the rows before planting so the spacing you paid for is the spacing that ends up in the soil. A depth mark scratched on the trowel helps in the same way, keeping planting speed up without guessing at every hole.

What your bulb spacing result means

The number is a purchasing figure first and a design figure second: it tells you what to order, and the density term tells you how the bed will read from ten feet away. Four bulbs per square foot looks generous in the package and perfect in the ground: bulbs are smaller than the space they occupy once the leaves fan out.

Depth matters as much as spacing for the following spring: planted too shallow, bulbs heave in freeze-thaw cycles and bake in summer; planted too deep in cold zones, they may not flower their first year while the bulb builds energy.

Compare geometric grid layouts against trench planting in our square foot vs intensive row gardening guide.

Common mistakes with the bulb spacing

• Planting every bulb at one depth: Measure tip to basal plate and multiply by four, because a large tulip and a small crocus need different depths to flower well. Mark each type before the first hole.
• Mixing inches of spacing with feet of bed: Convert both to the same unit before dividing; beds are measured in feet while spacing is quoted in inches, and mixing them skews every count. One conversion saves a second pass.
• Expecting bag counts to match bed maths: Round bags up and expect leftovers: curves and corners take a few extra, and a shortfall stalls the planting mid-row. One spare bulb per twenty planned covers losses and splits.
• Spacing for the package instead of the display: Choose density from the look you want from ten feet away, then let the total decide how many bags to order rather than buying whatever the budget allows at the counter.

Limits of the bulb spacing calculator

The grid assumes a rectangular bed and equal spacing: curves, edging and odd corners cost a few bulbs, which the spare recommendation covers. Counts round down to whole bulbs, so very tight spacings can differ by a handful from a pure area calculation. Bulb size varies by grade (a '12/14' tulip is a size range, not a single size), and depth guidance is for well-drained soil: bulbs rot in waterlogged ground no matter how carefully you space them.

Agronomy note from Dr. Dinesh A. Nagegowda, PhD: Flowering geophyte bulb spacing and planting depth adhere to the rule of thumb: planting depth must equal two to three times the vertical height of the bulb. In our horticultural trials, spacing bulbs too closely promotes stagnant microclimates and fungal Botrytis blight, whereas planting too shallow exposes bulbs to winter frost heaving and summer desiccation.

Ensure adequate organic matter depth before planting spring bulbs by calculating bed amendments with our garden compost volume calculator.

Sources

1. Colorado State Extension: Fall-Planted Bulbs and Corms (used for: planting about four times the bulb's height and large-bulb spacing on center)
2. Wisconsin Horticulture: Fall Bulb Planting (used for: large bulbs 6–8 in deep and small bulbs set 1–3 in apart on center)
3. Washington State Extension: Square Foot Gardening (used for: the square-foot grid layout used to place bulbs at a set spacing in a bed)

Bulb Spacing: frequently asked questions

More bulb spacing tools & guides

• Plant Spacing Calculator (Both pages lay out a bed on a grid: that one handles rows and in-row gaps for vegetables, while this one sizes bulbs and the depth they go in.)
• Mulch Calculator (The mulch page finishes the same bed, covering the planted bulbs with a measured layer once the depth this page returns has been used.)