When you fill your soap tank, do you know how much soap you’ll need for your washes? When you’re done washing a truck, do you know exactly how much product you used for that wash?

If you're like most operators, you might not.. And that's not your fault.

Many operators washing truck fleets rarely measure their soap usage. They fill the tank, connect the lines, and assume everything's working properly. The soap injector pulls what it pulls. The dilution is what it is.

When your soap dilution or draw rates aren't right, they could be causing soaps to work incorrectly, as well as potentially spotting & streaking surfaces. Worse,  you're either spending more time washing or more money on wasted soap. 

The 10-Minute Test For Accurate Dilution Ratios 

The first step is to find out what your equipment is doing in a real-world situation. All you will need is a measuring pitcher or a water bottle and your mobile phone. 

Use The Measuring Pitcher Method to Calculate Draw Rate

What you need:

• A measuring pitcher or metering jug with ounce markings
• Water
• A stopwatch (your phone works fine)

What to do:

1. Fill the pitcher with water to the top line
2. Put your soap pickup line in the bottle
3. Start your pressure washer and activate soap draw
4. Time yourself for 1 minute while drawing soap

This method works because it measures actual flow under real operating conditions - the same conditions your soap experiences during washing truck operations.

The simple math: Measure how much was drawn in a minute, here's your calculation:

• Top line (128 ounces) - ending line (96 ounces)
• 128 - 96 oz = 32 oz per minute

Use The Water Bottle Method to Calculate Draw Rate

What you need:

• A 20-oz water bottle
• Water
• A stopwatch (your phone works fine)

What to do:

1. Fill the bottle with water
2. Put your soap pickup line in the bottle
3. Start your pressure washer and activate soap mode
4. Time how long it takes to empty the bottle

The simple math: If it takes 37.5 seconds to drain a 20-oz bottle, here's your calculation:

• 60 seconds ÷ 37.5 seconds = 1.6
• 1.6 × 20-oz = 32-oz per minute

That's your draw rate — how much soap your injector pulls every minute while washing truck fleets.

Locate your GPM or use the Bucket Method

Now you need one more number: your pressure washer's GPM (gallons per minute). It's usually written right on the machine or the pump plate:

Can't find the GPM rating? Check the owner's manual or look up your model number online. Some older machines have worn labels, but it's worth finding. 

New or old, it is also valuable information to test your GPM. Your GPM is critical for accurate calculations. Testing GPM also helps to verify the real-world performance of your pressure washer to determine if maintenance or adjustments may be needed.

What you need:

• An empty rinsed 5-gallon bucket
• Preferably with a lid or small opening
• A stopwatch (your phone works fine)

What to do:

1. Start your pressure washer
2. Carefully aim your nozzle into the empty bucket
3. Try not to splash or spill
4. Time how long it takes to fill the empty bucket

The simple math: If it takes 75 seconds to fill a 5-gallon bucket, here's your calculation:

• 5 gallons / 75 seconds = 0.067 gallons per second
• 0.067 x 60 seconds per minute = 4 gallons per minute

That's your flow rate — how much liquid your pressure washer sprays every minute while in use.

Calculate Your Dilution Ratio

Here's the formula:

• Take your GPM and multiply by 128 (ounces in a gallon)
• Divide that number by your draw rate
• Subtract 1

Real example:

• 4 GPM pressure washer × 128 = 512 oz of water per minute
• 512 ÷ 32 oz soap draw rate = 16 total parts
• 16–1 = 15:1 dilution ratio

This means you're mixing 15 parts water with 1 part soap.

What Your Numbers Actually Mean

Different injectors and equipment setups have different draw rates. Here are some different ranges of draw rates you might have:

7:1 - 10:1 (High Draw)

• Heavy degreasing
• Stubborn road film
• “Soft washing” houses, buildings, and roofs
• High rate of soap consumption

10:1 - 20:1 (Average Draw)

• Ideal for most truck washing and equipment cleaning
• Good cleaning power without waste
• Target range for most injectors

20:1 - 40:1 & above (Low Draw) 

• Light maintenance washing only
• Best for high-pressure injectors 
• Requires more concentrated soap blends
• Weak draw rates can result in bad performance or re-washing
• Low soap consumption rates

Once you have determined your draw rate, adjust your chemical strength or mix ratios based on your range and the job you’re performing. There are also chemical metering systems and adjustable injectors to change your draw rate.

Quick Fixes for Common Problems

If your ratio is too strong (wasting soap):

• Check that you have the proper injector size for your GPM
• Switch to a smaller higher higher-pressure soap tip 
• Reduce draw rate with metering tips 
• Consider diluting your soaps to a weaker ratio to prevent overuse 

If your ratio is too weak (poor cleaning):

• Check that you have the proper injector size for your GPM
• Switch to a larger lower lower-pressure soap tip 
• Check for air leaks and clogged soap lines
• Consider using a more concentrated soap that is less diluted 

If results are inconsistent:

• Test your flow rate with and without the chemical injector running
• Chemical injectors create restrictions that can change your actual dilution
• Replace or repair worn injectors and soap components 

Take Action Today

You've got everything you need to test this right now. Grab a bottle & a bucket and spend 10 minutes finding out what your equipment is actually doing.

You might find you're using more soap than expected, or you might realize why your cleaning results vary from job to job.

Either way, you'll know your actual numbers instead of guessing.

Once you know your baseline, you can optimize your setup for consistent results and controlled costs.

If you are having trouble with your calculations or need help optimizing your setup, our technical team can walk you through these measurements. Hydro-Chem Systems has been solving pressure washing challenges for over 50 years. We can recommend the right approach for your operation.