Downstream Calculator
Pick your pressure washer and hose length and the calculator picks the injector, nozzle and soap tip for you, with an estimate of how hard that rig will draw. Then set the strength you want on the wall and your bleach tank size, and it tells you how many gallons of 12.5% sodium hypochlorite and water to put in the tank, plus how much surfactant to add to the bleach.
Built for downstreaming: a General Pump style injector on a 4 to 10 GPM pressure washer, pulling from a bleach tank. If you batch-mix for a 12V or gas soft wash pump instead, use the soft wash mix calculator. If you've bucket-tested your rig, enter your real draw and the calculator uses that instead of the estimate.
Downstream calculator
Pick your machine and hose length to get the right injector and an estimated draw. Then pick the strength you want on the wall and the calculator tells you how to mix the bleach tank.
Your downstream setup
- Injector—
- M5 Twist nozzle—low-pressure twist nozzle
- Soap tip size—J-Rod or shooter tip
- Estimated draw—
- 12.5% SH—
- Water—
Then add surfactant to the bleach
Surfactant goes straight into the bleach tank. It's figured per gallon of bleach, not per gallon of the whole tank.
The rest of the kit
- GP Hi-Draw fixed injector in the size above, around $20. Keep two spares made up with quick connects.
- 1/4" chemical filter on a 1/4" poly braid draw line, run through a 1/2" schedule 80 PVC riser tube so the filter stays submerged.
- Chemical-resistant ST-2315 house wash gun with the 6" lance and ceramic ball.
- M5 Twist downstream nozzle in the size above, or a stainless downstream J-Rod.
- Rinse control: move the drop stick to the buffer tank, add a manual bypass, or go remote with the Gen 5 FlowPro.
How the calculator figures your mix
A downstream injector pulls a roughly fixed amount of chemical, so the bigger the machine, the more water goes past it and the weaker the mix. The calculator estimates that draw from your GPM and hose length, then works backwards from the strength you want on the wall: tank strength = target % × (draw ratio + 1). A 4 GPM drawing about 9:1 needs a 10% tank to land at 1% on the wall, so 5 gallons is 4 gallons of 12.5% bleach and 1 gallon of water. When the math asks for a tank hotter than 12.5%, the target isn't reachable by downstreaming on that machine, and the calculator tells you the best you'll get with straight bleach instead of pretending.
The estimate is an estimate. Hose quality, fittings, nozzle, gun and the age of the injector all move it, so bucket-test your rig once (normal hose and soap tip on, one minute into a bucket, one gallon jug of water on the draw line) and type the numbers in. Everything above recalculates from your real ratio.
Which downstream injector for your pressure washer
GP Hi-Draw injectors are sized by the orifice inside, in millimeters. The number has nothing to do with draw ratio. The rule most of the industry runs on: match the body to your machine, then go one size smaller once your hose gets long, because the smaller orifice makes a stronger pressure drop and keeps pulling through the extra feet.
| Machine | Up to 150 ft of hose | 200 ft | 250 ft |
|---|---|---|---|
| Under 4 GPM | Not recommended. A 2.5 or 3 GPM can be made to pull with a 1.8 mm and a short hose, but the draw is unpredictable and the flow is too low to rinse a house in a reasonable time. A 4 GPM is the realistic floor. | ||
| 4 GPM | 2.1 mm | 2.1 mm | 1.8 mm |
| 5.5 GPM | 2.1 mm | 2.1 mm | 1.8 mm |
| 8 GPM | 2.3 mm (100 ft) or 2.1 mm | 2.1 mm | 2.1 mm |
| 10 GPM | 2.3 mm (100 ft) or 2.1 mm | 2.1 mm | 2.1 mm |
The 2.1 is the workhorse of the industry for a reason: it's on 4, 5.5 and 8 GPM machines alike. Don't go two sizes down. A 1.8 on an 8 GPM chokes the flow enough that you lose the reason you bought an 8 GPM. We don't recommend adjustable (dial-type) injectors at all; the fixed GP Hi-Draw costs around $20 and does the job better.
Typical draw ratio by machine size
Measured with a GP Hi-Draw through 200 feet of 3/8" hose and a low-pressure soap nozzle. Your rig will land somewhere in the range; the calculator uses the middle until you give it your bucket test.
| Machine | Typical draw | Straight 12.5% on the wall | What that means |
|---|---|---|---|
| 4 GPM | 7:1 to 10:1 | 1.1% to 1.5% | Draws strong. Cut the bleach with water for siding. |
| 5.5 GPM | 10:1 to 14:1 | 0.8% to 1.1% | Straight 12.5% lands right in house-wash range, a little stronger than an 8 GPM gets. Watch delicate surfaces. |
| 8 GPM | 15:1 to 20:1 | 0.6% to 0.8% | Run it straight. Dwell time and a second pass do the rest. |
| 10 GPM | 18:1 to 25:1 | 0.5% to 0.65% | Run it straight. Gutters and bad spots may want a pump-up. |
Bleach tank mixes for a 1% house wash
These are the calculator's answers for a 5 gallon bleach tank at the middle of each draw range. Scale them up for a bigger tank, or use the calculator and pick your tank size.
| Machine | 12.5% SH | Water | On the wall |
|---|---|---|---|
| 4 GPM (about 9:1) | 4 gal | 1 gal | 1% |
| 5.5 GPM (about 12:1) | 5 gal (straight) | 0 | about 0.95% |
| 8 GPM (about 18:1) | 5 gal (straight) | 0 | about 0.65% |
| 10 GPM (about 22:1) | 5 gal (straight) | 0 | about 0.55% |
If you want a milder mix on a 4 GPM, 3 gallons of water to 2 gallons of bleach is a common starting point and lands around 0.5% to 0.6% at the wall; it's gentle and cheap, and you can always step it up. On a 5.5 GPM, straight 12.5% is the normal starting point; it runs a little stronger than an 8 GPM gets from the same tank, so keep an eye on delicate surfaces and cut it 50/50 for a lightly soiled house. The wall tells you if you're right: green turning brown within a few minutes is good, instant sloughing is too hot, nothing after five to seven minutes is too weak or old bleach.
Surfactant: per gallon of bleach
Downstreaming surfactant goes straight into the bleach tank, and it's measured per gallon of bleach, not per gallon of the whole tank. Apple Suds and Lemon Suds run about 1 oz per gallon of bleach; Cherry Suds is double thick and runs about ¼ oz per gallon of bleach. All three have a booster built in that helps the bleach work better. Pour some bleach in first, add the surfactant, then top off so it doesn't foam up an empty tank. More is not better; too much soap means a house that takes forever to rinse.
The three ways to set up a downstream system
They all draw about the same. The difference is how much walking you do.
- Drop stick: a GP Hi-Draw injector between the whip line and the reel, a 1/4" poly braid draw line with a chemical filter, run through a 1/2" schedule 80 PVC riser tube so it stays submerged. Move the stick from bleach to the buffer tank to rinse.
- Manual bypass: a bypass assembly with a ball valve that routes water around the injector so the draw stops with a turn of the handle, and you get full flow for surface cleaning.
- Remote control: the Gen 5 FlowPro, a remote-controlled bypass you switch from your belt. It's the one we recommend and one of the most popular adds on our builds. Read how to set up the FlowPro.
On bypass and FlowPro setups we add a 1/4" three-way rinse valve with one hose in the bleach and one in the water, so at the end of the job you flip it to water and flush the injector without touching the drop stick.
Gun and nozzle
Bleach eats cheap guns until they restrict flow and kill your draw. We run the chemical-resistant ST-2315 house wash gun with a ceramic ball and a 6" stainless lance, and the M5 Twist downstream nozzle sized to the machine (3 to 7 GPM for a 4 or 5.5, 7 to 12 GPM for an 8). A stainless downstream J-Rod or a shooter tip does the same job with fixed tips. Bigger orifices draw more soap, not smaller; 0040 is the minimum soap tip for a 2.1 injector. Soap and rinse with the same low-pressure nozzle and stop the soap at the truck, not by switching to a high-pressure tip.
Keep reading
- Downstreaming with a pressure washer: the complete guide, including why your injector isn't pulling
- 4 GPM vs. 5.5 GPM vs. 8 GPM: what size pressure washer you actually need
- Pressure washing guns, J-Rods and the M5
- Soft wash mix calculator for batch mixing with a 12V or gas pump
- Fresh 12.5% bleach by the gallon at our Oregon City shop
Common questions
What ratio does a downstream injector pull?
It depends on the machine more than the injector. A GP Hi-Draw on a 4 GPM machine typically pulls 7:1 to 10:1 through 200 feet of hose; a 5.5 GPM pulls 10:1 to 14:1; an 8 GPM pulls 15:1 to 20:1; a 10 GPM pulls 18:1 to 25:1. The "20% draw" or "10:1" on the package is a best case, not a measurement. Bucket-test your rig to know yours.
How do I mix bleach for downstreaming a house?
Work backwards from the strength you want on the wall, usually 0.6% to 1% for vinyl and painted siding. Tank strength equals target percent times draw ratio plus one. On a 4 GPM drawing about 9:1, a 10% tank (4 gallons of 12.5% bleach to 1 gallon of water) lands at 1%. On 5.5 GPM and bigger machines, straight 12.5% is usually the answer, because the injector dilutes it enough on its own.
What size downstream injector do I need for an 8 GPM pressure washer?
A 2.1 mm (3 to 5 GPM body) once you're running 150 feet of hose or more, which is most rigs. A 2.3 mm (5 to 8 GPM body) is fine on a short hose run. Don't drop to a 1.8; it chokes the flow on an 8 GPM. A 2.1 on an 8 GPM whistles loudly, and that's normal.
Can I get a 1% mix downstreaming with an 8 GPM?
Usually not. An 8 GPM typically pulls 15:1 to 20:1, so straight 12.5% lands around 0.6% to 0.8% on the wall, and there's no injector or check valve that changes that physics. Run it straight, give it 5 to 10 minutes of dwell, and hit bad spots twice. If you truly need hotter than that, it's a 12V soft wash system, not an injector.
How much surfactant do I add when downstreaming?
Measure it per gallon of bleach in the tank, not per gallon of the whole mix. Apple Suds and Lemon Suds run about 1 oz per gallon of bleach; Cherry Suds is double thick and runs about ¼ oz per gallon of bleach. Pour some bleach in first, add the soap, then top off the tank.
How do I bucket-test my downstream injector?
Put your normal hose and soap nozzle on. Drop the draw line into a gallon jug of water, spray into a 5 gallon bucket for exactly one minute, then measure how much came out of the gun and how much left the jug. Water divided by chemical is your ratio. Enter both numbers in the calculator and it recalculates from your real draw.
Why does the calculator say my target isn't reachable?
Because the tank strength it would take is hotter than 12.5% bleach. Big machines dilute the injector's draw so much that straight bleach is the ceiling. The calculator shows you that ceiling instead of a mix that doesn't exist. Lower the target, lean on dwell time, or use a 12V soft wash pump for strengths over about 1%.
Should I dilute the bleach or run it straight?
Small machines dilute; big machines run straight. A 4 GPM draws strong enough that straight 12.5% is hotter than siding needs and wastes bleach, so cut it with water. A 5.5 GPM is right on the line and usually runs straight. 8 GPM and up always run straight because the draw is thin by nature.
