Theoretical and effective capacity
Field capacity measures how much ground a machine covers per hour. The theoretical capacity assumes it always works at full width and speed:
Acres per hour = speed (mph) × width (ft) ÷ 8.25
The constant 8.25 comes from 43,560 ft² per acre divided by 5,280 ft per mile. In practice, time is lost turning at headlands, overlapping passes, filling seed or spray tanks, unloading grain and making adjustments. The field efficiency accounts for these losses:
Effective capacity = theoretical capacity × field efficiency
Example
A 30 ft planter at 5 mph has a theoretical capacity of 5 × 30 ÷ 8.25 ≈ 18.2 acres per hour. With a field efficiency of 75%, it actually covers about 13.6 acres per hour, so a 160-acre field takes about 11.7 hours.
Typical values
| Operation | Field efficiency | Typical speed |
|---|---|---|
| Tillage (disk, field cultivator) | 75% to 90% | 4 to 8 mph |
| Planting | 50% to 75% | 4 to 8 mph |
| Spraying | 50% to 80% | 6 to 15 mph |
| Mowing hay | 75% to 85% | 3 to 8 mph |
| Combining grain | 60% to 75% | 2 to 5 mph |
These ranges are based on ASABE data; your numbers depend on field shape, size and logistics.
What improves field efficiency
- Larger, rectangular fields with long passes reduce the share of time spent turning.
- Guidance and auto-steer cut overlaps, often by 5% to 10%.
- Logistics: a grain cart or nurse tank keeps the main machine working instead of waiting.
- Matching equipment: a wider implement only helps if the tractor can pull it at the planned speed.
Using the result
Field capacity helps you plan how many days a job will take in the available weather windows, compare machines of different widths, and estimate labor and fuel per acre. Multiply the hours by the fuel use per hour to estimate fuel per field, or divide the machine’s hourly cost by the effective capacity to get a cost per acre.
Measuring your own efficiency
Record the total time in the field and the acres covered for a few days, then divide the actual rate by the theoretical one. Tracking it each season shows where the time is going.