Downstream injector calculator

Enter your injector's draw, the way you know it, and the strength in the bucket. See the percent of sodium hypochlorite at the nozzle and how fast the bucket drains. Or enter the strength you want and get the bucket mix.

About 1%Straight 10% SH on a 10:1 draw

4 waysRatio, percent or bucket test

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Strength at the nozzle

1.14%

12.5% in the bucket × 9.09% draw

9.09%Chemical share

0.4 gal/minBucket use

12.5 min5 gal lasts about

Makers' draw ratings are best cases. Long hose and an injector sized for a different flow cut the draw. 3R Sales says their 10% injector draws easily through 100 ft of 3/8 in. hose and often works to 200 ft. A bucket test gives your real number. Read the SH label before you mix or spray; for a registered product, the label directions are the law. Wear eye protection, chemical gloves and the respirator the SDS calls for. Never put acids, ammonia or other cleaners in the same bucket or line as chlorine. Wet plants first and rinse them after, and keep runoff out of storm drains as your local rules require.

How we got this

Draw1 ÷ (10 + 1) = 9.09% of what leaves the nozzle is chemicalTwo readings of a ratio are in use. As parts, 10:1 is 1 part chemical to 10 parts water, 11 parts in all. Many washers read 10:1 as a tenth of the bucket strength. Pick the one your injector's maker means, or do a bucket test.
At the nozzle12.5% × 9.09% = 1.14%
Bucket use4 gpm × 9.09% ÷ 90.91% = 0.4 gal a minute

Draw	Nozzle %, read as parts	Nozzle %, read as 1/R
5:1	2.08%	2.5%
8:1	1.39%	1.56%
10:1	1.14%	1.25%
12:1	0.96%	1.04%
15:1	0.78%	0.83%
20:1	0.6%	0.63%

Sources: Kleen-Rite, J.E. Adams 7263P downstream injector, 2.1 orifice, 10% draw; Kleen-Rite, J.E. Adams 7264P downstream injector, 2.3 orifice, 10% draw, 5 to 8 gpm; Kleen-Rite, General Pump 100776 hi-draw injector, 2.3 mm orifice, draw up to 20%; 3R Sales and Service, Downstream soap injector, 10% fixed (hose length guidance); Pressure Washing Resource forum, Chem dilution chart (10:1 and straight 10% SH gives 1% at the tip); Pressure Washing Resource forum, Why not a 3 to 1 downstream injector (the bucket test); Pressure Washing Resource forum, Downstream query (real draws near 10:1, about 1% max from 10% SH); PHTA Recreational Water Quality Committee, Sodium hypochlorite tech notes, AQUA May 2020 (12.5% half-life 180 days at 77°F, 48 days at 95°F); EPA-accepted label, Sodium Hypochlorite Solution 12.5%, EPA Reg. No. 55852-3 (PPE, mixing hazards, degrades with age). Checked 2026-10-08.

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Key takeaways

Strength at the nozzle = strength in the bucket × the share of the output that is chemical. Straight 12.5% SH through a 10% draw is 1.25%.
A 10:1 ratio is read two ways: 1 part in 11 (12.5% becomes 1.14%) or a tenth of the strength (1.25%). A bucket test settles which, and what your injector really pulls.
Makers' draw ratings are best cases. Long hose and the wrong nozzle or orifice cut the draw.

How a downstream injector sets your mix

A downstream injector sits after the pump. When you switch to a soap nozzle, pressure drops, water rushing through the injector's small orifice makes a vacuum, and that pulls chemical up the pickup tube from your bucket. A check valve stops water running back into the bucket when you go back to high pressure.

The injector does not know or care what is in the bucket. It pulls a share of the flow and mixes it with the machine's water. So the math is short:

Strength at the nozzle = strength in the bucket × chemical share of the output

If 10% of what leaves the nozzle came from the bucket, and the bucket is straight 12.5% SH, the nozzle sees 1.25%. If you cut the bucket to 6.25%, you get 0.63%. To get more than the injector allows, you cannot just make the bucket stronger than the SH you bought. That ceiling is why downstreaming suits house washes and other low-strength jobs.

The quick checkStraight SH on a 10:1 injector gives roughly a tenth of its strength at the nozzle: about 1% from 10% SH, about 1.1% to 1.25% from 12.5% SH.

What 10:1 means: two readings

Injector draws are quoted as ratios (10:1), as percents (10% draw) or in plain words. The trouble is that a ratio gets read two ways in the trade, and the answers differ by about a tenth.

As parts. 10:1 is 1 part chemical to 10 parts water, 11 parts in all. The chemical share is 1 ÷ 11, or 9.1%. This is how a mixing ratio normally works.
As a fraction. Many washers read 10:1 as a tenth of the bucket strength. On the Pressure Washing Resource forum a member explains it as 10 gallons out of the washer for every gallon drawn, so straight 10% SH comes out at 1%. The chemical share is 1 ÷ 10, or 10%.

The calculator does both. Use whichever your injector's maker means, or skip the argument and do a bucket test.

Draw	12.5% SH, read as parts	12.5% SH, read as a fraction	10% SH, read as parts	10% SH, read as a fraction
5:1	2.08%	2.5%	1.67%	2%
8:1	1.39%	1.56%	1.11%	1.25%
10:1	1.14%	1.25%	0.91%	1%
12:1	0.96%	1.04%	0.77%	0.83%
15:1	0.78%	0.83%	0.63%	0.67%
20:1	0.6%	0.63%	0.48%	0.5%

These are for straight SH in the bucket. Cut the bucket and every figure drops in proportion.

Rated draw and real draw

Injector makers list draw as a percent. Two common sizes, as listed by Kleen-Rite:

Injector	Orifice	Machine flow	Rated draw
J.E. Adams 7263P, fixed	2.1	3 to 5 gpm	10%
J.E. Adams 7264P, fixed	2.3	5 to 8 gpm	10%
General Pump 100776, hi-draw	2.3 mm	5 to 8 gpm	Up to 20%

Makers list a flow range for each injector, so match it to your machine. Then expect less than the rating. On the Pressure Washing Resource forum, washers report that injectors sold as 20% draw mostly pull closer to 10:1 in practice, and that about 1% is the most they get with fresh 10% SH. What cuts the draw:

Hose length. More hose means more back pressure. 3R Sales says its 10% injector draws easily through 100 ft of 3/8 in. hose and often works with up to 200 ft.
The nozzle. The injector only draws once pressure drops, so it needs a soap nozzle or an adjustable or dual lance set to low pressure. 3R Sales lists these as what makes a downstream injector work.
Weak SH. Not a draw problem, but it looks like one. SH loses strength in storage: PHTA puts the half-life of 12.5% at about 180 days at 77°F and 48 days at 95°F.

How to do a bucket test

A bucket test measures what your rig really pulls, with your hose, your nozzle and your machine. It takes five minutes and settles the ratio question.

Set up exactly as you work: same hose length, same soap nozzle, injector at the same spot.
Put the pickup tube in a jug with a known amount of water in it, for example 1 gallon. Use water for the test, not SH.
Spray into an empty 5 gallon bucket until it is full, then stop.
See how much left the jug.

Share = gallons drawn ÷ gallons caught at the nozzle. A forum member puts it simply: if the 1 gallon jug empties in the time it takes to fill the 5 gallon bucket, you are drawing 1 in 5. That is a 20% share, so straight 12.5% SH would give 2.5% at the nozzle. If the jug only dropped half a gallon, the share is 10% and you get 1.25%.

Choose "My bucket test" in the calculator and enter the two numbers. Redo the test when you change hose length or nozzles.

Worked examples

Straight 12.5% SH, 10:1 read as parts. Share = 1 ÷ 11 = 9.09%. At the nozzle: 12.5 × 0.0909 = 1.14%.

A 0.75% house wash on a 10% draw. The bucket needs 0.75 ÷ 0.10 = 7.5%. In a 5 gallon bucket from 12.5% SH: 5 × 7.5 ÷ 12.5 = 3 gallons of SH and 2 gallons of water.

How long does the bucket last? On a 4 gpm machine, a 10% share means 4 × 0.10 ÷ 0.90 = 0.44 gallons a minute from the bucket, so 5 gallons lasts about 11 minutes of spraying. On an 8 gpm machine it lasts about half that.

3% for concrete. With 10% SH on a 10:1 draw, straight SH reaches only 1% at the nozzle. No bucket mix fixes that, because the bucket cannot be stronger than the SH. For stronger mixes, washers use a higher-draw tip such as an X-Jet, a 12 volt or soft wash pump, or a pump sprayer with a batch mix. The soft wash mix calculator does the batch.

Target at the nozzle	Bucket strength on a 10% share	12.5% SH in a 5 gal bucket	Water
0.5%	5%	2 gal	3 gal
0.75%	7.5%	3 gal	2 gal
1%	10%	4 gal	1 gal
1.25%	12.5%	5 gal (straight)	None
1.5% or more	Over 12.5%	Not possible	Use a higher draw

Downstreaming or batch mixing

A downstream injector is the cheapest way to soft wash: no extra pump, no tank, just a bucket and the pressure washer you already own. Its limit is strength. With 10% to 12.5% SH and a draw near 10:1, the most it can put on the wall is roughly 1% to 1.25%. That covers most house washes, which suppliers start around 1%.

Where it falls short:

Roofs. Suppliers start asphalt shingle roofs around 3%. A downstream injector cannot get there with ordinary SH, and roof makers warn against pressure washing shingles at all.
Concrete pre-treat. Suppliers give about 3% to 4%, again out of reach.
Exact strength. The draw changes with hose length, nozzle and machine flow, so the strength at the wall drifts during the day. A batch mix in a tank is the same strength from the first gallon to the last.

You can do both: downstream the house wash, and batch mix the roof and flatwork mixes in a tank or sprayer. When you batch mix, the strength at the wall is the strength in the tank, and the soft wash mix calculator gives the gallons of SH and water for it.

Whichever you use, write the mix on the job. If a wall comes out streaky or a customer asks what you sprayed near their plants, you will want to know the bucket strength, the draw and how old the SH was.

Safety and the label

The label governsFor an EPA-registered product, using it in a way its label does not allow is a violation of federal law. Read the SH label and the safety data sheet before you mix or spray. This calculator does the dilution math only.

PPE. The 12.5% SH label calls it corrosive and says to wear safety glasses and rubber gloves. Add the respirator and clothing the SDS calls for.
One chemical per line. The label warns that SH mixed with ammonia, acids or detergents gives off chlorine gas. Never run an acid product through the same bucket, pickup or injector without flushing it, and never mix them in one bucket.
Plants and runoff. Wet plants before and rinse after. The label says SH is toxic to fish; keep runoff out of storm drains and follow your city's wash water rules.

To price the job, use the pressure washing cost calculator, then send it as an estimate from Koira.

Last checked October 8, 2026. More for your trade: how pressure washing companies run on Koira.

FAQ

What percent of SH do I get from a downstream injector?

Bucket strength times the draw share. Straight 12.5% SH through a 10% draw gives 1.25% at the nozzle; read as 1 part in 11, it gives 1.14%. Straight 10% SH gives about 1%.

What does a 10:1 downstream injector mean?

About 1 part chemical for every 10 parts of water. Read strictly as parts, that is 1 in 11 in total. Many washers read it as a tenth of the bucket strength. A bucket test tells you what yours really pulls.

Can I get 3% at the nozzle with a downstream injector?

Not with a normal injector and 10% to 12.5% SH. At a 10% draw the most you get is about 1% to 1.25%. Stronger mixes need a higher-draw tip, a 12 volt or soft wash pump, or a batch mix in a sprayer.

How do I do a bucket test on my injector?

Draw water from a jug with a known amount in it while filling a 5 gallon bucket at the nozzle. Divide the gallons drawn by the gallons caught. Emptying 1 gallon while filling 5 is a 20% share.

Why is my downstream injector not pulling chemical?

Common causes are too much hose, an injector rated for a different flow than your machine, or a high pressure nozzle instead of a soap nozzle. A downstream injector only draws once the pressure drops.

How much SH do I put in the bucket for a house wash?

Divide the strength you want by the draw share. For 0.75% at the nozzle on a 10% draw, the bucket needs 7.5%: 3 gallons of 12.5% SH and 2 gallons of water in a 5 gallon bucket.

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