How much soap/detergent is optimal to wash your clothes?

Lifehacks Asked by Stan on July 22, 2020

What is the best way to determine how much detergent to use to clean my clothes in any average washing machine.

Some of the suggested guidelines I’ve come across are:

The published (detergent manufacturer’s suggested) method is to measure for a normal load to the slightly raised line molded into the cap for the detergent container (by volume).
For a heavy (dirty) load, fill to the second mark molded into the cap.

The published (washer manufacturer’s suggested) method is to load the washer tub loosely to a mark on the agitator after you have added the detergent (determined by the above) to the empty washer tup before loading.


Often, people have gotten on the bus or into a subway car and the strong odour of their clothing fills the car within seconds of their arrival. They reek of the detergent. Evidently, the existing odour from an excessive amount of the detergent is associated with ‘clean’ as opposed to chemically ‘contaminated.’ The brand, notwithstanding. Just sayin’

When I wash my clothes, I need enough detergent/soap to get the dirt out of my clothes; but, without using an excess which is wasteful and environmentally destructive.
How can I determine the correct/optimal amount of detergent to use in any situation given that the optimal amount is affected by:

  • The wash load weight
  • The load volume
  • The kind of fabric
  • The amount and kind of ‘dirt’ in the fabric
  • The water hardness/softness
  • The water temperature
  • The concentration of the detergent
  • The washing machine itself?

After the dirty-work is done, I want the wash water to take the suspended material away in the rinse which under expected conditions should be part of normal operation for the make and model.

Summary: Long story short: I want to determine the minimum effective detergent concentration during the wash cycle, in real time, not after the fact.

Hint: If one added excessive detergent, say, what would indicate such an oversight before the wash cycle is complete?
If one forgot to add detergent, what would indicate such an oversight before the wash cycle is complete?
— Somewhere between those two extremes is the answer. It should not matter if sweatpants used in mud wrestling is in one load and an equivalent weight of grand-mother’s crochet table runners is in the next.

Is there a lifehack?

Thank you.

One Answer

This will get you in the ballpark:

  1. Use a graduated cylinder to measure how much detergent is in the manufacturer's recommendation.

  2. wash one load of clothing, of the maximum weight allowed in your machine, and with an average level of dirt, using the manufacturer's recommendation.
    A full load means the machine works most efficiently, and makes the process more repeatable.

  3. Wash the next load using 90% of the manufacturer's recommendation. After washing, check if the clothes are sufficiently clean (no visible stains or weird smells).

Repeat step 3 on subsequent loads, reducing the amount by 10% each time until it fails your check. That gives you a minimum amount of detergent to use. This process necessarily ignores some factors (exact level and type of dirt, type of fabric), but it's as close as you'll get without an exhaustive analysis of each load. That would require a chemical laboratory and a day of analysis for each load of laundry, which is clearly inefficient.

To minimize the smell, use non-perfumed detergent or choose a brand that's hypoallergenic.

There are washing machines that can do this for you during one cycle, instead of having to check the result afterwards.

They use a combination of inputs:

  • the amount of laundry (several ways to do this, either load cells to measure the weight directly, or it may be possible to do torque sensing on the motor and derive the weight from that)
  • turbidity sensor in the drain, this measures how much dirt ends up in the drain

They use these inputs to adjust not only the amount of detergent, but the entire washing cycle. Replicating this functionality is not a life hack, it'd be a major electronics and software development project, replacing half the parts in the machine.

To add these to a machine that doesn't have them, you'd need to replace the machine's computer, replicate its programming, and modify it to use the new sensor inputs. You'd have to figure out how the sensors work (e.g. what sensor reading corresponds to which level of dirt) and how this should modify the machine's program. An advanced electronics hobbyist would take months to implement this.

Or you can spend a few hundred dollars more on your next washing machine.

They reek of the detergent.

I suspect those people just dump a load of detergent into the washer rather than follow the manufacturer's specification, and end up using far more detergent than necessary. They don't need a life hack, they need to RTFM and follow the instructions therein.

Answered by Hobbes on July 22, 2020

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