How many yards in a tri-axle?
As many as the body holds, unless the dirt is heavy enough to reach the payload first. Light dirt fills the body; wet clay, wet sand and rock stop at the payload with room to spare. The table uses the example tri-axle (16 loose yd³ heaped, 18 tons) and the middle of each material's published weight range.
| Material | Loose yd³ | Bank yd³ | Tons | Limited by |
|---|---|---|---|---|
| Common earth (loam or silt), dry | 16.0 | 12.8 | 16.4 | Body |
| Common earth (loam or silt), wet | 12.0 | 9.6 | 18.0 | Payload |
| Clay, dry | 16.0 | 11.4 | 14.8 | Body |
| Clay, wet | 13.3 | 9.5 | 18.0 | Payload |
| Sand, dry | 13.1 | 11.7 | 18.0 | Payload |
| Sand, wet | 12.2 | 10.9 | 18.0 | Payload |
| Gravel, dry | 12.6 | 11.3 | 18.0 | Payload |
| Gravel, wet | 12.2 | 10.9 | 18.0 | Payload |
| Shale (soft rock) | 14.1 | 8.6 | 18.0 | Payload |
| Trap rock, blasted | 11.6 | 7.7 | 18.0 | Payload |
Loose yards per load in the other example road trucks:
| Material | Tandem14 yd³ / 13 t | Tri-axle16 yd³ / 18 t | Quad-axle18 yd³ / 21 t |
|---|---|---|---|
| Common earth (loam or silt), dry | 12.7* | 16.0 | 18.0 |
| Common earth (loam or silt), wet | 8.7* | 12.0* | 14.0* |
| Clay, dry | 14.0 | 16.0 | 18.0 |
| Clay, wet | 9.6* | 13.3* | 15.6* |
| Sand, dry | 9.5* | 13.1* | 15.3* |
| Sand, wet | 8.8* | 12.2* | 14.2* |
| Gravel, dry | 9.1* | 12.6* | 14.7* |
| Gravel, wet | 8.8* | 12.2* | 14.2* |
| Shale (soft rock) | 10.2* | 14.1* | 16.5* |
| Trap rock, blasted | 8.4* | 11.6* | 13.5* |
* Limited by payload. The truck sizes are examples, not legal limits; put your own trucks in the calculator.
How the calculator works
- Bank to loose. Loose yd³ = bank yd³ × (1 + swell). Dirt measured in place grows when it's dug: dry clay swells about 40%, so 100 bank yards fill 140 yards of truck.
- Weight. Tons = loose yd³ × loose weight (lb/yd³) ÷ 2,000.
- One load. A truck carries its body full, or its payload, whichever comes first: load = the smaller of the heaped body and payload × 2,000 ÷ loose weight. Most free calculators fill trucks by volume alone, which overloads them in heavy material.
- Loads. Loads = loose yd³ ÷ loose yd³ per load, rounded up.
- Loading time. Minutes to load one truck = bank yd³ per load ÷ excavator output (bank yd³/hr) × 60.
- Round trip. (Loading + haul miles ÷ loaded mph + haul miles ÷ empty mph + dump and spot) ÷ (working minutes ÷ 60). A 50-minute hour stretches every cycle by a fifth.
- Trucks. Trucks to keep the excavator loading = round trip ÷ loading time, rounded up. The US Army's earthmoving manual adds one truck so one is always waiting under the bucket (FM 5-434, para. 10-10); the calculator shows both.
- Hours. The spread moves the excavator's output, or what the trucks can carry if there are too few of them: trucks × bank yd³ per load ÷ round trip. Hours = bank yd³ ÷ that rate.
The round trip and the truck count use the same functions Site Bid prices trucking with, so the numbers here match what Bid would carry into an estimate.
A worked example
1,500 bank yd³ of clay, dry (40% swell, 1,850 lb per loose yd³) hauled 6 miles each way in tri-axles (16 yd³, 18 tons), loaded by an excavator doing 150 bank yd³ an hour, at 30 mph loaded and 35 empty, with 8 minutes to dump and spot and a 50-minute hour:
- Loose: 1,500 × 1.40 = 2,100 yd³, weighing 1,943 tons.
- One load: 16.0 yd³ is 14.8 tons, under the payload, so the body limits it. That's 11.43 bank yd³.
- Loads: 2,100 ÷ 16.0 = 132.
- Loading: 11.43 ÷ 150 × 60 = 4.6 minutes.
- Round trip: 41.8 minutes, including the 50-minute hour.
- Trucks: 41.8 ÷ 4.6 = 9.15, so 10 trucks, or 11 with one always waiting.
- Time: 1,500 ÷ 150 = 10.0 hours, about 1.1 9-hour days.
Choosing the numbers
Swell and weight. The material list uses the US Army's published averages, set at the middle of each range. They vary with grain size, moisture and how tight the ground is, so a scale ticket from your own pit or your own haul beats any table. Wet material is heavier, which is why wet clay reaches a tri-axle's payload before it fills the body.
| Material | Swell | Loose lb/yd³ |
|---|---|---|
| Common earth (loam or silt), dry | 15 to 35% | 1,900 to 2,200 |
| Common earth (loam or silt), wet | 25% | 2,800 to 3,200 |
| Clay, dry | 40% | 1,700 to 2,000 |
| Clay, wet | 40% | 2,400 to 3,000 |
| Sand, dry | 10 to 15% | 2,600 to 2,900 |
| Sand, wet | 10 to 15% | 2,800 to 3,100 |
| Gravel, dry | 10 to 15% | 2,700 to 3,000 |
| Gravel, wet | 10 to 15% | 2,800 to 3,100 |
| Shale (soft rock) | 65% | 2,400 to 2,700 |
| Trap rock, blasted | 50% | 2,700 to 3,500 |
Payload. What a road truck may carry is its allowed gross weight less its empty weight. The allowed gross depends on the truck's registration, the spacing of its axles and the roads it travels. In North Carolina the limits, and the exceptions for trucks hauling dirt, sand, rock and aggregate, are in G.S. 20-118. The truck sizes in the calculator are examples to start from.
Excavator output. Use what the machine really loads in this material, in bank yards an hour, not its book rating. If you know your cycle instead: bucket (yd³) × fill factor × passes an hour gives loose yards an hour; divide by (1 + swell) for bank yards.
Speeds and the 50-minute hour. Average speeds over the whole road, including traffic lights and the haul road on site, are usually well under the posted limit. The working-minutes allowance covers the delays every cycle has; 50 minutes is the usual starting point.
Sources
- Swell, loose weights and the trucks-needed rule: US Army FM 5-434, Earthmoving Operations (2000), Table 1-2 and paragraph 10-10. Approved for public release. Read the manual. Checked October 3, 2026.
- North Carolina vehicle weight limits: G.S. 20-118. ncleg.gov. Checked October 3, 2026.
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