Ballast Coverage Calculator
Ballast for a run of track, traced as the trapezoid it actually is — a flat-slab estimate comes up more than a quarter short.
Bulk density is the weight of a scoop of ballast including the air between the grains, not the density of the rock — so weigh a measured container of your own rather than trusting a figure for granite. This is the geometric volume, with nothing added for what ends up on the floor. Nothing is uploaded.
The profile is a trapezoid, and the shoulders are not a rounding error
Ballast is not a flat slab: it falls away on both sides. Each shoulder is a triangle whose base is the slope times the depth, so together they add the slope times the depth squared — a term that grows faster than the depth does. At 0.1875 in it is 22% of the ballast you buy.
| Depth | Flat estimate | Real profile | Shoulder is | Flat estimate is short by |
|---|---|---|---|---|
| A sixteenth | 0.0625 sq in | 0.0684 sq in | 8.6% | 9.4% |
| Three thirty-seconds | 0.0938 sq in | 0.1069 sq in | 12.3% | 14.1% |
| An eighth | 0.1250 sq in | 0.1484 sq in | 15.8% | 18.8% |
| Five thirty-seconds | 0.1563 sq in | 0.1929 sq in | 19.0% | 23.4% |
| Three sixteenths | 0.1875 sq in | 0.2402 sq in | 22.0% | 28.1% |
| A quarter | 0.2500 sq in | 0.3438 sq in | 27.3% | 37.5% |
Those last two columns are the same shoulder measured against different things, and they are not the same number — at 0.1875 in they are 22% and 28%. The one to order by is the shortfall, because that is what a flat estimate leaves you needing. Quoting the share of the total instead understates the error, always.
And the depth nobody writes down moves the answer most
Going from an eighth of an inch to three sixteenths is one and a half times the depth — and 1.62 times the ballast, because the shoulder term is squared. A change nobody would call significant, made without thinking, buys sixty percent more stone.
| Depth | For ten feet of track | Weight | Against a sixteenth |
|---|---|---|---|
| A sixteenth | 8.2 cu in | 0.45 lb | 1.00× |
| Three thirty-seconds | 12.8 cu in | 0.71 lb | 1.56× |
| An eighth | 17.8 cu in | 0.98 lb | 2.17× |
| Five thirty-seconds | 23.1 cu in | 1.27 lb | 2.82× |
| Three sixteenths | 28.8 cu in | 1.58 lb | 3.51× |
| A quarter | 41.3 cu in | 2.27 lb | 5.03× |
A quarter inch is four times the depth of a sixteenth and 5.0 times the ballast. The length of track is the easy half of this calculation — it is exactly proportional, and it is the number everyone remembers to state. The depth is neither, and it is the one left out of the question.
How to use
- Enter the feet of track you are ballasting.
- Set the top width and the depth of your profile.
- Set how far the shoulder falls away per unit of depth.
- Read the volume, the weight, and what a flat estimate would have missed.
Frequently asked questions
How much ballast do I need per foot of track?
For a typical HO profile — an inch wide, three sixteenths deep, shoulders at 1.5 to 1 — it is about 2.9 cubic inches a foot, which is roughly 0.16 lb at a bulk density of 95 lb per cubic foot. Ten feet comes to about a pound and a half.
Why is my estimate always short?
Because ballast is not a flat slab. It falls away on both sides, and each shoulder is a triangle of base slope times depth. Together they add slope times depth SQUARED, which at three sixteenths makes a flat estimate 28% short.
Is the shoulder really worth bothering with?
It grows with the depth, so it depends where you are. At a sixteenth of an inch it adds 9%, which you could ignore. At a quarter inch it adds 37%, which you cannot — that is most of a second bag.
Why do two shoulder percentages get quoted?
Because they use different denominators. The shoulder is 22% of the ballast you buy and the flat estimate is 28% short — same stone, different question. Order by the shortfall; the other figure is always the smaller of the two and will leave you under.
How much does the depth matter?
More than anything else, and it is the number nobody writes down. Going from an eighth to three sixteenths is one and a half times the depth and 1.62 times the ballast, because the shoulder term is squared. A quarter inch is five times a sixteenth.
What bulk density should I use?
Weigh your own. Bulk density counts the air between the grains, so it is well below the density of the rock and varies with the grain size and how it settles. Fill a measured container, weigh it, and divide — that takes a minute and removes the guess.
Should I buy extra?
Yes. This is the geometric volume of ballast in place, with nothing added for what ends up on the floor, in the vacuum, or stuck to a glue-covered finger. Treat the figure as a floor and add a comfortable margin.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.