Cost per wear is the price divided by the number of times you will actually wear it, and the whole exercise lives or dies on that second number. A 600 pound coat worn twice a week for four winters costs about two pounds a wear. A 90 pound dress worn once costs 90. That is the entire argument for buying less and better, and it is also the argument people use to justify things they will never wear, so it needs a rule attached.

Mine is that I have to be able to name the three occasions. Not three categories, three actual occasions in the next two months. If I cannot, the number is fiction. What follows are the three places where the maths reliably works out, and the pieces I would put money into on that basis.

Below, the three pieces where cost per wear is easiest to prove: the coat you wear over everything, the knit that holds its shape, and the trouser you will actually rewear. Nili Lotan, Tibi, Khaite and A.L.C. are in, and Joseph takes the Label Love.

Outerwear is where cost per wear is most obviously true, because a coat goes over every outfit you own and is seen more than anything else in your wardrobe. In a cold climate a good coat can genuinely be worn a hundred times a year.

The things that decide whether it lasts. Check the shoulder fit first, because a coat is tailored from the shoulder and that is the one thing you cannot alter cheaply. Look for a wool content above about 70 percent for warmth and drape, a lining that is fully finished rather than half, and a length that clears whatever you actually wear underneath, since a coat that stops mid-skirt will annoy you every day. Rails and Anine Bing are the accessible end here, Jenni Kayne and St. Agni the middle, and Khaite and Nili Lotan the once-a-decade purchases.

Knitwear is the second best cost per wear category and the one most often wasted, because the failure is not that you stop liking it, it is that it stops looking good. A jumper that pills, sags at the cuffs or stretches at the shoulders drops out of rotation long before it wears out.

So I buy for structure rather than softness. A firm rib at the cuff and hem, a shoulder seam that sits where your shoulder is, and a two-ply yarn rather than a single. Fold it rather than hanging it and the shape survives years longer. ATM Collection and White & Warren are the reliable middle, Joseph is the best value for real cashmere in this grid, and Jenni Kayne and Tibi are the ones I would buy if the plan were to keep it for a decade. Ganni's wool pieces are the least precious and the easiest to wear daily.

Trousers are the sleeper category. Nobody photographs them, everybody wears them constantly, and a good pair quietly does more work than any dress you own.

What makes a pair rewearable. A fabric with a little recovery, so cady, twill, or a wool blend rather than a loose weave that seats out. A rise that stays put when you sit, which you can only test by sitting. A hem you can have altered, which means enough fabric turned inside. And a colour you own shoes for. Joseph's comfort cady trousers are the most rewearable thing in this issue and cost less than most of the tops. Splendid and Cinq À Sept sit at the everyday end, La Ligne and Cult Gaia at the considered end. If you buy one pair, buy it in the colour you already wear on your feet.

Joseph is here because it is the clearest example in this issue of a house whose whole proposition is cost per wear. The coats, the trousers and the knitwear are the three categories above, and they are the three things Joseph has made for decades.

The range splits usefully. The cashmere and merino knits at the lower end of the price list are, to my mind, the best value real cashmere among the brands in this issue. The tailoring is cut long and lean and does need trying on. The leather and shearling pieces are the top of the range and priced accordingly. What I would actually buy: a cashmere cardigan and a pair of the gabardine or cady trousers, which together are less than one of the coats and will be worn far more often. Sizing is European and runs slim through the hip.

The pieces earning their place on my list this month.

ATM Collection cashmere crew neck in midnight, the plainest useful thing here.

St. Agni cropped leather trench in khaki, an outer layer that will outlive the trend.

Hammitt Shelley tote, the bag I would take to work daily.

Cult Gaia Farim blazer in off white, the one indulgence.

Cinq À Sept short sleeve Khloe blazer, for a hot office.

Rails Jac blazer in black, the most wearable jacket price on the list.

Tibi Herbert loafer, a flat that goes with the trousers above.

White & Warren cashmere linen blend cable button cardigan, the three-season knit.

The questions I ask before I do the maths.

Name the three occasions. Real ones, in the next two months. If you cannot, the cost per wear number is imaginary.

Check what it needs. A piece that requires shoes, a bra or a coat you do not own is not one purchase, it is three, and the maths has to include them.

Count what it replaces. If it does the job of something you already wear, you will split the wears between them rather than doubling them.

Alter it or return it. A coat or trouser that needs a hem is fine. A shoulder that does not fit is not fixable.

If you take one thing away: spend on the things that go over and under everything else, which is coats, knitwear and trousers, and be sceptical of anything that only works with one other item you own. The wardrobe maths only ever works on the pieces that combine.