How to Choose, Calibrate and Use a Knapsack Sprayer
A knapsack sprayer is the tool a pump sprayer can't keep up with once you're doing a whole drive, not a whole path. Here's how to pick the right tank size and type, how to calibrate it so you're actually applying the rate the label expects, the common faults and their real fixes, and the licence question answered straight.
The short version
A hand-pumped 1-5 litre pressure sprayer suits a few stray weeds; anything the size of a driveway wants a 15-16 litre knapsack worn on the back, manual lever or battery-powered. There’s no licence to own or use the sprayer itself, calibration is just matching how much you spray to a measured area, and most “it won’t spray” problems are a blocked filter or nozzle from residue left to dry in it.
Sprayer types, and what each is actually for
Match the tank size to the job before you match the brand. A 5 litre pressure sprayer on a full drive means refilling and re-pumping a dozen times over.
| Type | Best for | How it works | Verdict |
|---|---|---|---|
| 1-5 litre pressure/pump sprayer | A few weeds, cracks, a small patio | Pump up by hand, spray until pressure drops, pump again | Cheap and light, but you’ll be re-pumping constantly on anything bigger than a path. |
| 15-16 litre manual knapsack (lever-operated) | A whole drive, a long run of block paving | Worn on the back, lever pumped with one hand while you spray with the other | The trade standard. No batteries to charge, cheap to maintain, and you can feel when pressure is dropping. |
| 15-20 litre battery-powered knapsack | The same jobs, less arm work | Electric pump holds a constant pressure, you just squeeze the trigger | Even output all the way down the tank, which makes a consistent mix easier to apply evenly. Battery life and a motor that can fail are the trade-off. |
The nozzle matters as much as the tank
Most knapsacks ship with an adjustable cone nozzle that twists from a fine mist to a solid jet. For weedkiller, you want the coarser end of that range, not the finest mist it can produce. Professional glyphosate labels specify application as a “medium or coarse spray” (the standard BCPC droplet classification) at nozzle pressures not exceeding 2.5 bar, and that’s not a technicality: a fine mist drifts further in any breeze, meaning more of what you paid for ends up on the plants either side of the weed you’re aiming at, not the weed itself.
A flat-fan nozzle tip, sold separately for most knapsack models, gives a more even band of coverage along a straight run, a path edge or a fence line, than the cone nozzle’s circular pattern does. It’s a small upgrade that’s worth it if most of your spraying is long straight lines rather than odd patches.
Calibrating it: how to know you're applying the right rate
Every weedkiller label states its dose as a rate per hectare in a stated volume of water, for example one widely-used professional glyphosate label gives its hard-surface rate as 2.9 litres of product in 250 litres of water per hectare, which works out to one litre of made-up spray solution treating roughly 40 square metres. That figure is the whole point of calibrating: it tells you how far your tank should actually go, so you can check your sprayer is putting down that much, not more and not less.
To check it yourself: fill the tank with plain water, mark out a measured area (a driveway is easy, most are 30-50 square metres), and spray it at your normal walking pace and nozzle setting exactly as you would with the real mix. Measure how much water you used to cover that area, then work out the equivalent per-hectare rate and compare it to the label’s. Walking faster or opening the nozzle wider both mean less product lands per square metre than you think; walking slower or a finer mist means more. Get that dialled in with water before you ever mix the real thing, and the same setting is reusable every time you refill.
Common faults, and the actual fix
Won’t spray at all or loses pressure fast: check the inline filter and the nozzle tip first, dried weedkiller residue left in a sprayer between jobs is the single most common cause of a blockage. On a manual lever model, pressure dropping as you go and needing a few more pumps mid-job is completely normal, not a fault. If a manual model won’t build any pressure at all even when pumped, the pump seal or diaphragm has likely worn out and needs replacing.
Uneven spray pattern or dribbling from the nozzle: usually a partially blocked nozzle or a worn nozzle washer letting the pattern collapse. A cheap spares kit (washers, filters, a spare nozzle tip) fixes most of these faults for a few pounds rather than a whole new sprayer.
The mechanism seizes up between seasons: almost always down to storing it with old mix or rinse water still inside. Empty and rinse it fully at the end of every single session, before it goes back in the shed.
Cleaning, care and what to wear
Rinse the sprayer straight after every use: half-fill the tank with clean water, agitate it, then pump the rinse water through the lance and nozzle to flush the whole system, and do it twice. Manufacturer guidance on professional glyphosate products is explicit that traces of herbicide left in a sprayer can seriously damage or destroy whatever is sprayed with the same equipment later, so if you ever use the sprayer for anything else, a plant feed, a fungicide, treat that rinse as non-negotiable rather than optional.
For PPE, professional label guidance for hand-held sprayer use calls for suitable protective gloves, protective clothing (coveralls) and rubber boots when handling concentrate and spraying, plus washing hands and any exposed skin before eating and after finishing. That’s written for the concentrated professional product, but it’s sound practice with any glyphosate concentrate, gloves on for mixing at minimum.
About this resource
I’m The Weed Guy, one bloke who’s qualified to spray professional weedkiller. This resource reflects the equipment knowledge and label-reading that comes with that, not a comparison test of specific sprayer models.
The honest disclosure: nothing here is a paid-for opinion, no brand sponsors this site, and there are currently no affiliate links on this page. If that ever changes it will be disclosed plainly. The verdicts are by sprayer TYPE and tank size, which is what actually determines whether it’s the right tool for your job, not a brand comparison I have no honest basis to make.
Common questions
Do you need a licence to use a knapsack sprayer?
There’s no general licence for owning or using a knapsack sprayer itself; it’s just a tool. What can require certification is the weedkiller you put in it. HSE guidance is clear that anyone using a plant protection product authorised for professional use must hold the relevant Certificate of Competence (or be working under the direct, personal supervision of someone who does) if they were born after 31 December 1964, and this applies whether you’re spraying commercially or on your own land. Amateur/consumer-labelled weedkillers, the kind sold in garden centres and DIY stores, carry no such requirement and can be bought and used by anyone.
What’s the difference between a knapsack sprayer and a backpack sprayer?
Nothing, they’re the same thing. "Knapsack" is the older, more common term in UK gardening and amenity trade circles; "backpack sprayer" is the more literal, more common term in retail listings. Both mean a tank worn on the back with a lance and trigger, as opposed to a smaller pressure sprayer you carry in one hand or set down on the ground.
Why won’t my knapsack sprayer spray, or why has it lost pressure?
Three usual suspects. First, a blocked filter or nozzle, weedkiller residue dries into a paste if left in the mechanism, so check the inline filter and unscrew the nozzle tip to look for a blockage. Second, on a manual lever model, pressure genuinely does fall as you spray and needs topping up with a few more pumps, that’s normal, not a fault. Third, a worn pump seal or diaphragm on an older sprayer will stop it building pressure at all, and that’s a parts-replacement job, not a clean.
How do I clean a knapsack sprayer after using weedkiller?
Rinse it straight after use, not the next time you reach for it. Half-fill the tank with clean water, agitate it, then pump it through the lance and nozzle to flush the whole system, and repeat once more. This matters beyond keeping the sprayer working: if you ever use the same sprayer for anything else, from a plant feed to a fungicide, dried herbicide residue left in the tank or lines can carry over and damage or kill whatever you spray next.
Manual or battery-powered, which is better for weedkiller?
Manual lever knapsacks are the simpler, cheaper, more repairable choice, and they’re what most people spraying a driveway or a path actually need. A battery-powered knapsack holds a constant pressure without you pumping, which gives a more even application rate from the first litre to the last, useful if you’re doing a large area and want a consistent result without the pressure dropping off toward the end of the tank. Neither changes what weedkiller you use or how much of it, that’s a mixing-rate question, not an equipment one.
How much weed killer do I actually put in a knapsack sprayer?
That depends entirely on the concentration of the product you’ve bought, not on the sprayer, every label states its own rate and it varies a lot between an amateur concentrate and a professional one. See the full mixing-rate breakdown, with worked examples for common tank sizes, on the glyphosate concentrate and mixing rates page.