Greywater Volume Calculator

When people think of greywater, they picture a maze of pipes and filtering units, all hidden beneath garden surface. In most cases, it’s far less complicated, and begins by asking one simple question: How much greywater does your house generate?

If you don’t have an answer to that, you can’t calculate the right-size tank… Or distribution system. That means estimating based off your routine. What happens in your own home?

How to Calculate Your Greywater

Here’s a tool to run the numbers for you. But first, understand where the inputs come from, and you’ll see your own home in a new way.

If you’ve ever considered diverting greywater, here’s something important to know. Greywater comes from laundry, showers, baths, and bathroom sinks, but not kitchen or toilet water. The boundary between greywater and other types of wastewater is importent. Why? Because greywater is typically cleaner than blackwater, which includes human waste and associated pathogens.

Kitchen water, in particular, is usually full of food particles, fats, and oils that will quickly clog an irrigation system. Keeping the greywater definition narrow protects against the all-too-common error of over-optimizing your assumptions about what might be safely diverted to your non-potable reuse system. The tool preserves this boundary, and makes sure your output reflects water that’s actualy usable for reuse in most jurisdictions.

The biggest variable for most households are showers. How long do you take them? What’s the flow rate? This all sounds dry until you think about what one fixture change can make. Replacing your old three-and-a-half gallon per minute showerhead with a two-gallon per minute WaterSense version reduces amount of water used by almost forty percent.

That’s not just a number on your water bill; that’s less water your greywater system has to process. And if four people is taking showers each day, that translates into dozens of gallons you’re saving each morning. The table near the top of the page makes that clear; it also shows how those decisions will add up over time.

Baths are another problem since they’re not continuous flow events but instead batch events. Based on size of your tub and how deep you like to get in, you may be using up to 36+ gallons just filling it up. If you bathe maybe once or twice a week, this still represents a pretty big spike in your weekly volume.

To plan with, the calculator smooths out that variation and turns those weekly baths into an average per day. That way you can see the peak load you’ll need to accommodate and make sure your system doesn’t overflow. It will also show you where a bigger view (weekly) might be better than a daily one for sizing distribution or storage components.

But that’s only part of the laundry story. Today’s Energy Star washers consume far fewer gallons of water (sometimes down to only fourteen) per load compared to previous years’ standard models.

But here’s where chemistry of what you put in it also comes into play. Even with a small amount of water, bleach and other heavy softeners is harmful to plants and soil organisms. This makes it off-limits to most greywater uses, though this varies by location. Some jurisdictions don’t allow any laundry greywater at all for this reason; always check local laws.

You’ll customize the tool based on your kind of washing machine, but that provides a realistic starting point, and then you’re free to fret over the chemistry.

The other little bit that always dribbles on is bathroom sinks. Hand washing for three minutes each per day by everyone in the house don’t seem like much, but it adds up quickly throughout the home. Add that into the calculation at the standard planning rate of one-and-a-half gallon per minute and you have another entry in the overall pot to consider.

In rough estimates, it’s easy to forget to add this factor but doing so gives you a better picture of what resources you can work with.

And the output? A number of gallons per day and week (and liters too). You can base your plans on creating a subsurface irrigation setup, a system that flushes toilets, or a rain garden.

This is what helps you know how much water you’re using now. And whether it will match your plan to reuse some portion of it.

Perhaps you realize that your biggest culprit is all those showers, or maybe it’s those extra-large loads of laundry. Now that you have hard facts instead of assumptions, you can begin to plan accordingly. You can go from something that seems like a lofty ideal, conservation, to something concrete. Turn an abstraction into something you can measure.

Water stops being merely a commodity. It becomes a resource with quantity, velocity, and value. And the purpose isn’t saving so much as knowing it.