Idea File: Garden calculations for the math challenged

How much compost is in this bag?
Today’s Idea File is really an “I-Have-No-Idea File” – for when someone asks you how much mulch or manure or compost or topsoil you need to cover an area, and you answer, “I have no idea!”

This is especially tricky in Canada, where we straddle metric and imperial measurements. If you measure your garden in feet, at the garden centre you’ll discover that they often sell manure (or mulch or compost or topsoil) in litres or kilos. Even with all metric, this linear-to-volume conversion can make your eyes rattle.

I’ll focus on the feet-to-litres calculations here, because it’s so oddball. Later, you’ll find link that takes you to garden math formulas for many other purposes and measurements.

First, find the area of your garden beds:

So: if your planting area is a single rectangle, it’s simple to measure its length and width, then multiply the two to get the total area. If it’s 10 feet long by 6 feet wide, that’s an area of 60 square feet. The same calculation (length x width = area) works in metric, too.

If you have more than one garden bed, calculate the area of each bed, then add the areas together to get your total area.

Next, calculate the volume of soil amendments you’ll need:

To add a 3″ layer of manure on top of your beds:

  • Multiply the total area by 3 (if you’re adding 2″ or 4″, use that number instead).
  • In my example, 60 square feet times 3 inches is 180.
  • Now that isn’t cubic feet, but remember that number for your next calculation. (area x depth in inches = remember that number!)

Ordering manure (or compost or topsoil) by the cubic yard, like the big bag pictured above, can be a little less expensive and can also give you a wider range of media to choose from. My bag contained duck compost, which isn’t commonly available.

To order that much in a bag like the orange one:
  • To translate that remembered number into cubic yards, divide by 324.
  • In my example, 180 divided by 324 rounds up to roughly 0.6 cubic yards, or a little more than half one of those big orange cubic yard bags. If that’s too much for your garden, consider splitting a bag with your neighbour.
OR to buy that much in bags at the garden centre:

On the other hand, you can go to the garden centre and buy your manure (or mulch or compost or topsoil) in individual bags – and that’s where we run into that cubic feet/yards to litres/kilograms conundrum.

Commercially bagged soil amendments often come in 30L/15kg sizes. To calculate the number of bags in my example:

  • Go back to the 0.6 cubic yards (remembered number divided by 324)
  • Then use this volume converter from MetricConversion.org to turn it into litres.
  • In my example, that gives you about 459 litres.
  • If the bags come in 30-litre sizes, divide total litres by 30 (or the number of litres per bag)
  • In my example, I’d need to get about 15 bags.
Other garden math tools:

If you want an even better roundup of math tools like this, for all sorts of garden calculations, check out Garden Math from Washington State University. I referred to it when I wrote my post, but any errors are mine. [UPDATE: In March 2019, this link no longer works, but here’s a substitute from extension.org]

And, for bonus points, Alex – how many plants to buy:

When you’re covering large areas with plants such as groundcovers, you need to know how many plants to buy. If you get excited by pi – not the kind that comes with ice cream – you might try this accurate plant spacing calculator from Landscape Architecture Resource.

On the other hand, if you prefer to generally guesstimate, here are handy plant spacing tables from Brian Wilson at Gardenality.

Goodnight, good luck and happy gardening!

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