Two terms, and the second one is the wild card
The force needed to move a load along a slope is the weight multiplied by the sine of the slope angle, plus the weight multiplied by a resistance coefficient multiplied by the cosine of the slope angle. The first term is gravity pulling the load back down the hill. The second is the ground refusing to let it roll or slide. Written out: force equals weight times the quantity sine theta plus Cr cosine theta.
The gravity term is exact. If you know the grade, you know that part of the answer, and on a 15 percent grade it is about 14.8 percent of the weight. The resistance term is not exact and never will be, because Cr describes an interaction between a wheel or a hull and a surface that varies from moment to moment. On pavement a rolling vehicle sits around 0.02. On firm dirt it is perhaps 0.10 to 0.15. In sand or soft going, 0.2 to 0.3. In deep mud it can be 0.3 to 0.4 and worse. Dragging something that is not rolling at all is a different regime again, commonly quoted in the region of 0.5 to 0.8 on dirt.
Between the ends of that list is a factor of twenty or more. On a 5,000 lb load, the ground resistance ranges from about 100 lb on pavement to over 3,000 lb being dragged. Which is to say: the slope term is arithmetic and the surface term is judgement, and anyone who quotes a single pulling force without stating a surface has not said anything useful.
What a snatch block actually does
A snatch block is a pulley that the line runs around, redirecting it back toward an anchor near the winch. The load is then supported by two parts of line instead of one, so each part carries roughly half the resistance and the winch sees roughly half the pull. In exchange, the winch has to spool in twice as much line, and the load moves at half the speed the drum is turning.
The halving is not exact. A sheave on a bearing loses a few percent to friction, so the real reduction is somewhat less than half — the calculator uses an efficiency figure to show this, and a dirty or worn sheave is worse than the default. More importantly, the block and its anchor are carrying the sum of both line parts, which is more than either the line or the winch sees. That means the block, the shackle attaching it, and whatever it is anchored to are the most heavily loaded items in the whole arrangement, and each of them has a rating that has to be checked rather than assumed. A block rated for the winch pull is not necessarily rated for what the block carries.
Three practical consequences follow. Doubling the line takes the winch onto a lower drum layer, where its rated pull is higher, so the benefit compounds. Halving the speed makes a marginal pull possible that a fast one was not, because the motor is not being asked for as much at once. And the extra rigging adds another set of components, every one of which is a new thing that can fail.
The rated pull is not the pull you have
| What reduces available pull | Why |
|---|---|
| Line layers on the drum | Rated pull is on the bottom layer. Each layer increases the drum radius and reduces torque at the line, often substantially by the outer layers. |
| Duty cycle | Winch motors are rated for short pulls. Heat builds fast and performance falls with it. |
| Battery and cabling | A winch under load draws enormous current. A tired battery, a small alternator or undersized cable all cost pull. |
| Line condition | Damaged wire rope or abraded synthetic is not at its rating, and neither is line that has been shock-loaded. |
| Fairlead and angle | Pulling off to one side loads the fairlead, spools line unevenly and stacks it on one side of the drum. |
The manufacturer publishes a per-layer table for the winch. Use that table rather than the default in the field above, which is only there so the calculation has something to work with. The practical habit is to spool line off before starting a heavy pull so that you are working on the first or second layer with the pull you actually paid for.
What this page will not do
It gives a number. It does not tell you how to rig, and that is deliberate rather than an oversight. A line under tension holds a large amount of stored energy and releases it along its own axis if anything in the chain lets go — line, hook, shackle, block, anchor, or the recovery point on the vehicle. Wire rope and synthetic line fail differently and neither of them is safe to be near. A kinetic recovery strap is a completely different tool, designed to stretch and release energy deliberately, and it does not belong anywhere near a winch line or a static pull. A tow ball is not a recovery point, and balls that have been used as one have come off and killed people.
None of that is something to learn from a page. Recovery is taught in person, with the equipment in front of you, by somebody who can see what you are about to do. What is worth taking from here is the sense of scale: that a bogged 5,000 lb vehicle on a modest slope can need several thousand pounds of pull, that the number moves enormously with the surface, and that everything holding that force needs a rating you have actually read. If you are working out whether something will drive onto a trailer under its own power instead, that is the loading ramp calculator, and once it is on, the cargo securement calculator and the tongue weight calculator take over. For the vehicle end of the trip, the roadside emergency guide and the outdoor safety guide are the right context.
Questions people ask
How big a winch do I need for my vehicle?
The common advice is to size the winch above the vehicle weight by a comfortable margin, and the reason that advice exists is everything on this page: the pull required depends on the slope and the surface far more than on the vehicle weight alone, and a figure that works on a firm track is nowhere near what a mud-filled ditch asks for. Work the arithmetic for the worst case you actually expect — the steepest grade, the softest ground, the vehicle loaded — and then compare it against the winch pull on the drum layer you will really be on rather than the number on the box. Then remember that a stuck vehicle can need a peak force well above the steady-state figure to break free at all. Sizing conversations are worth having with people who recover vehicles for a living rather than settling from a calculation.
Does a snatch block really double my winch capacity?
It roughly halves the line pull the winch has to produce for a given resistance, which is not quite the same claim. The reduction is a little less than half because the sheave has friction, and the price is that the winch spools twice as much line and the load moves at half the speed. There is a second, less obvious benefit: doubling the line keeps the winch on a lower drum layer for longer, and the rated pull is higher on lower layers, so the arrangement helps twice. The thing to be careful about is that the block and its anchor carry the sum of both line parts, which is more than either the winch or the line sees. Every item in that anchor chain needs a rating that covers it, and the anchor itself — a tree, a stake, a vehicle — is the part nobody rates.
What rolling resistance coefficient should I use?
Pick from the ranges and then err toward the higher end, because the consequences of underestimating are worse than the consequences of overestimating. Rolling on pavement is around 0.02, gravel around 0.05, firm dirt and grass in the region of 0.10 to 0.15, soft ground and sand 0.2 to 0.3, deep mud 0.3 to 0.4 and beyond, and something being dragged rather than rolled 0.5 to 0.8 on dirt. These are broad planning figures rather than measured constants — the real value depends on tire pressure, tread, wheel size, how deep the wheels have sunk and what the ground is doing that day. If a wheel is in a hole, or the load is suctioned into mud, or a tire is flat, none of these numbers apply and the force to start it moving can be several times the force to keep it moving.
Why is the pull needed to get it moving higher than the pull to keep it going?
Because starting and moving are different problems. A load that has been sitting has settled, and mud in particular develops suction that has to be broken before anything moves at all. A wheel in a depression has to climb out of it, which is a much steeper local slope than the average grade. Static friction is generally higher than the resistance to continuing motion. And a vehicle with a flat tire, a locked brake or a damaged wheel is not rolling at all and belongs in the drag category rather than the rolling one. All of that means the steady-state force this page calculates is a floor, not a peak. The peak is what the equipment actually has to survive, and it is the reason margin matters more here than in most calculations.
Can I use a tow strap on my winch line to reach further?
No, and this is one of the specific mistakes that gets people killed. A kinetic recovery strap is engineered to stretch under load and release that energy to snatch a vehicle free, which is a completely different job from a slow static pull. Putting one in a winch line means storing a large amount of energy in an elastic member with a winch steadily adding more, and when something in the chain fails the strap returns that energy along the line with whatever is attached to its end. Winch line extensions are a specific product with a specific rating and no stretch, and that is what belongs there. The same reasoning covers what the line attaches to: a tow ball is a spherical piece of steel held by a nut and it is not a recovery point regardless of how convenient it is. This is exactly the territory where the right move is to get trained rather than to read.