Understanding the One In, One Out Strategy
When I was still new to minimalism and sponging up as much advice as I could find, I came across no shortage of blog posts and articles about decluttering with the "one item in, one item out" strategy. Anyone who understands basic math will realize that a 1:1 discard-to-new-stuff ratio isn't decluttering at all. What's more is that it is not a good starting point for new minimalists, and trying to "declutter" this way will only end up in frustration. Of course, it's not all bad, and I'll explain the bad as well as the good with this particular method.
When One In, One Out is not the best strategy
One In, One Out is not a helpful strategy for someone who's just starting a decluttering or minimalism journey. To meaningfully declutter, we need to get rid of more stuff than we bring back in--plain and simple. There will come a point when each minimalist has decluttered and discarded enough that they have reached the right amount of things to have, but that's hard to achieve by maintaining status quo. There's a particularly bad habit that can inadvertently spring from One In, One Out--basically a reversal to One Out, One In, so that any time you get rid of something you feel compelled to replace it. When discarding and decluttering for the first time, that absolutely cannot be the case if you hope to make good progress.
The most important thing to do when decluttering and discarding for the first time is to not bring in any new items as you take out old items. I'll echo Marie Kondo and strongly advise against buying or bringing in any new organizing systems, tempting as they might be. Take the stacking bins and utility shelves out of your online and brick-and-mortar shopping carts until your decluttering is complete. My own advice on top of Marie's is to not bring in replacements for any discarded items right away. Try to wait between two weeks and a month after discarding something to determine if a replacement is really necessary. If it's a matter of having discarded something that broke from regular use, it will make sense to replace. But if three bags of clothing just made their way to a donation drop-off, it does not make sense to bring in any new clothes that might just suffer the same fate as the discards.
If you're new to minimalism and your end goal is really to have fewer things, set yourself up for success and ignore the temptation of the One In, One Out strategy until you're happy with how much you have.
When One In, One Out is a good strategy
Although I don't like the strategy for budding minimalists, I will sing the praises of One In, One Out for established minimalists. One In, One Out is a good way to maintain equilibrium of "want" items. I find that clothing and most consumables are the best targets for 1:1 replacements, but every minimalist will find their own good 1:1 categories. In my case, I keep a lean closet by having my own sort of uniform and capsule wardrobe, and only replace garments as they wear out or stop fitting well; and as a wine drinker and minimalist, I keep my bar stocked with 6 bottles of red wine, as that's all my wine rack holds and I despise clutter (even if it's delicious clutter).
One In, One Out is also a great strategy for keeping quantities of "need" items in check. While it can be tempting to opt for a 6-pack case of dish detergent to save on price-per-unit, keeping just one or two under the 1:1 replacement model means I'll still have space under the sink for other kitchen cleaning products. The same goes for foods, bathroom supplies, and so on. Making 1:1 replacements has the benefit of keeping storage spaces uncluttered so it's easier to find, grab, and use what you need as you need it.
There is a lot of benefit to a 1:1 replacement strategy once you've got an understanding of how much of any one thing is right for you. One In, One Out should be avoided early on when dipping your toes into decluttering efforts or a minimalist lifestyle so that you can make meaningful progress and keep up momentum on improving your surroundings. Once you know how much is enough, though, the 1:1 replacement strategy will help maintain a happy equilibrium. You'll know One In, One Out is working for you when seeing the quantity
of items you have, versus the storage space available, is not a
stressor. And of course, even if you're a seasoned minimalist, you can always take more than one out for every one in!