Remember water at 1 kg per litre and wheat at about 750 kg/m³ and you can estimate most cargo: a 20-litre bucket holds about 20 kg of water, 15 kg of grain and 30 kg of gravel. Loose gravel at 1,400-1,800 kg/m³ is much lighter than solid stone at about 2,200-2,800 kg/m³, and a green softwood log 2 m long and 25 cm across weighs 79-98 kg, not the 40-60 kg of a seasoned one. To use a weight in a structure, convert it to newtons by multiplying kilograms by 9.81.
Part of Logistics & Load Planning
Reference densities of the materials a settlement moves or builds with, and how to turn a volume into a weight and a weight into a force.
Weight Is Volume Times Density, And Density Varies By Condition
Every load estimate is the same sum: volume in cubic metres times density in kilograms per cubic metre. Density is not one number per material. Wood is twice as heavy green as seasoned, stone is lighter broken into loose rubble than as a solid block, and grain shifts with moisture. The parent article, Logistics & Load Planning, gives a reference table; here it is set out with the units and two corrections.
| Material | Density |
|---|---|
| Water | 1 kg per litre (1,000 kg/m³) |
| Green (wet) wood | 800-1000 kg/m³ |
| Seasoned hardwood | 600-800 kg/m³ |
| Seasoned softwood | 400-500 kg/m³ |
| Loose stone or gravel | 1400-1800 kg/m³ |
| Dry sand | 1500-1700 kg/m³ |
| Wheat or corn | about 750 kg/m³ |
| Loose hay | 60-80 kg/m³ |
| Iron or steel | about 7800 kg/m³ |
| Clay bricks | 1800-2000 kg/m³ |
| Potatoes and root vegetables | about 700 kg/m³ |
The parent also lists compressed hay at 150-200 kg per bale. That is a weight per bale, not a density, and it does not say how big the bale is; weigh a bale before planning with it.
Solid stone is not on the parent’s list. The row for stone and gravel, 1400-1800, is for loose material, which has air between the pieces. Material Properties gives solid granite at 165-175 lb/ft³, limestone at 140-170 and sandstone at 120-145. At 16.02 kg/m³ per lb/ft³ that is about 2,640-2,800, 2,240-2,720 and 1,920-2,320 kg/m³; granite and limestone together span about 2,200-2,800 kg/m³. Use these for a block, a slab, an arch or an abutment, and the 1400-1800 figure for a heap of rubble in a cart.
What You Need
- A tape to measure volume.
- A bucket of known size, such as the 20-litre bucket.
- A reference weight, for example a litre of water.
- A way to weigh a sample. See Improvised Weighing.
Method: From Volume To Weight To Force
Step 1: Get the volume For a box: length × width × height. For a log: π × radius² × length. For a heap: estimate it as a cone or a prism and work in cubic metres.
Step 2: Multiply by density Use the range, not a single point. Give the answer as a range.
Step 3: Convert to a force if it will load a structure Force in newtons = mass in kilograms × 9.81. A mass of 1,000 kg exerts 9,810 N.
Step 4: Weigh a sample if the stakes are high Fill a known bucket, weigh it, and scale up.
Worked Example: Bucket Shortcuts, A Log And A Cart Bed
The bucket shortcuts check out. 20 litres is 0.020 m³. Water: 0.020 × 1,000 = 20 kg. Wheat: 0.020 × 750 = 15 kg. Gravel: 0.020 × 1,400 = 28 kg to 0.020 × 1,800 = 36 kg, so about 30 kg.
The log shortcut does not. The parent says a log 2 m × 25 cm across weighs about 40-60 kg and calls it green softwood. Volume = π × 0.125² × 2 = 0.0982 m³. The parent’s own densities then give:
- Green wood at 800-1000 kg/m³: 0.0982 × 800 = 79 kg to 0.0982 × 1,000 = 98 kg.
- Seasoned softwood at 400-500 kg/m³: 0.0982 × 400 = 39 kg to 0.0982 × 500 = 49 kg. So 40-60 kg is the weight of a seasoned log. A green one is roughly twice that. This matters when you lift it or count it as the dead load of a bridge.
A cart bed. The parent article Carts and Wagons gives 600-1,200 kg for a two-horse cart. Take a bed of 2.0 m × 1.2 m = 2.4 m² and a 600 kg limit. Depth at which each material reaches 600 kg:
- Wheat: 600 / (750 × 2.4) = 0.33 m.
- Loose gravel at 1,800: 600 / (1,800 × 2.4) = 0.14 m; at 1,400: 600 / (1,400 × 2.4) = 0.18 m.
- Loose hay at 80: 600 / (80 × 2.4) = 3.1 m; at 60: 4.2 m.
Gravel hits the weight limit at a depth of only 14-18 cm, so a cart of gravel looks nearly empty when it is full. Hay is the opposite: it would take 3.1-4.2 m of loose hay to reach 600 kg, so volume and height limit the load long before weight does, and a tall hay load is top-heavy. See Cart & Wagon Loading.
Common Mistakes
- Using the loose-gravel density for a stone block — it understates a solid granite block by up to half (1,400 against 2,800).
- Treating a seasoned-wood weight as green — it is half the real number.
- Mixing kilograms and newtons — kilograms for planning loads, newtons for forces in a structure.
- Using one value rather than a range — density varies with moisture and packing.
Safety
Underestimated loads break equipment and backs
A load that weighs double the estimate overloads the axle, the animal and the lifters. Weigh a sample of anything you will lift by hand or load on a cart, and do not lift more than two people can handle. A green log of 79-98 kg is a two-person lift.
What’s Next
- Learn next: Improvised Weighing — checking an estimate
- Combine with: Cart & Wagon Loading — placing the weights on the vehicle