Capacity De-Rate Calculator
Rated capacity assumes a centered, balanced load on un-extended forks, with outriggers deployed, on a level hard surface, indoors. Real conditions reduce that capacity. This calculator applies the engineering de-rates described in Sumner's operator's manuals to give you a defensible working number.
Educational reference, not engineering certification
This calculator implements de-rate rules from Sumner's published operator's manuals and standard rigging-engineering practice. It is not a substitute for OEM guidance on your specific lift, and it does not replace a qualified rigger's judgment for non-routine lifts.
Adjusted Safe Working Load
650 lb
Rated 650 lb · 100%
✓ Within design margin
Why this load was de-rated
Methodology
How the de-rates are calculated.
| Factor | Range | Effect on capacity | Source |
|---|---|---|---|
| Fork extension kit installed | Yes / No | Hard cap at 200 lb regardless of base capacity | Sumner PN 783691 spec |
| Off-center load | 0 – 18 in | Linear fall-off, ~5.5% per inch (conservative; a real rigger treats any offset > ~3 in on an extended mast as a re-rig, not a de-rate — offset penalty scales with mast height) | Tipping-moment rule of thumb |
| Floor slope | 0 – 8° | ~8% per degree of slope | Sumner manual: hard, level surface only |
| Outriggers not deployed | Boolean | ×0.4 (60% reduction) | Footprint-based stability calculation |
| Wind exposure | None / Light / Strong | ×1.0 / ×0.7 / ×0.4 | Sumner manual: do not operate in wind |
| Mast extension | 0 – 100% | Up to 10% reduction at full extension | Dynamic margin for slide-pad slop and sway |
The de-rates are multiplicative
Multiple factors compound. Off-center load on a slope with outriggers stowed isn't the sum of three penalties — it's the product. A 650 lb lift can drop below 100 lb safe working load very quickly when several factors stack.