Quick answer: Buy 1/2 pound of raw potatoes per person (8 oz / 225 g) for a holiday side. That yields about 1 cup, or 7–8 oz (200–225 g), of finished mash. Ten guests need 5 lb — one standard bag. Go to 3/4 lb per person if you want leftovers.
Half a pound of raw potatoes per person. That is the number to shop with, and once peeled, boiled and mashed with butter and milk it comes out at roughly one cup on the plate.
Two numbers get quoted for this question and they look close enough to be the same one. They are not. One is the weight of potatoes in the bag; the other is the weight of mash in the dish. Nobody labels which they mean, and that gap is where the shopping goes wrong.
Two units, both unlabelled
Omnicalculator, which holds the top spot for this query, works in finished mash: "a typical serving size is 2/3-1 cup or 6-8 oz." It treats one cup as roughly 7 ounces.
Almost everyone else works in raw potatoes. Parade and The Takeout both say "⅓ to ½ pound of potatoes per person"; Southern Living: "The rule of thumb is a 1/2 pound of potatoes per person."
Six ounces of mash and a third of a pound of potatoes (5.3 oz) read as the same quantity. They describe different objects at different points in the process, and the one site that noticed said so honestly. Eat or Toss wrote: "once you add butter and milk, a pound of potatoes might yield more than a pound of mashed potatoes (though I'm not sure how to calculate the weight lost from peeling potatoes–this remains an ongoing investigation)." That investigation is the whole of this section.
The conversion, done properly
Two things happen between the bag and the bowl, and they pull in opposite directions.
Peeling takes weight off. The Idaho Potato Commission is blunt: "Peeling the skin on a potato will drop your yields by 20% over leaving the peels on." A pound in the bag is about 12.8 oz of usable potato.
Butter, milk and cream put weight back on. The clearest evidence is the USDA's standardised mashed potato recipe for schools, which states both its inputs and its output: 12 lb of unpeeled Yukon Golds, 1 qt of 1% milk and 12 oz of margarine yield "about 14 lb," or 50 half-cup servings. That is a 17% weight gain, from a recipe lean by holiday standards.
Peel the potatoes, use a home cook's butter and cream, and the two effects very nearly cancel. Running the USDA numbers back through a 20% peeling loss gives roughly 1 lb of raw potatoes → 2 cups → 14–16 oz of mash — matching the only other source willing to publish a conversion, Summer & Cinnamon: "1 pound of raw potatoes yields about 2 cups of finished mashed potatoes."
| Raw potatoes | Finished mash | Serves (1 cup) |
|---|---|---|
| 1/3 lb (5 oz / 150 g) | 2/3 cup · 5 oz (140 g) | 1 light |
| 1/2 lb (8 oz / 225 g) | 1 cup · 7–8 oz (210 g) | 1 |
| 3/4 lb (12 oz / 340 g) | 1½ cups · 11 oz (310 g) | 1 + leftovers |
| 1 lb (16 oz / 450 g) | 2 cups · 15 oz (425 g) | 2 |
| 5 lb bag (2.3 kg) | 10 cups · 4½ lb (2 kg) | 10 |
Which way the mistake runs
Buy 6 oz of raw potato per head because a calculator said "6 oz" and you land at about three-quarters of a cup on the plate — roughly 20% short of the half-pound standard. Read it the other way and you over-buy by the same margin. At ten guests that is one bag either way, which is survivable.
But the error stays small only because two mistakes cancel, not because the units convert, and the cancellation breaks at the edges. Skin-on mash finished with heavy cream gains weight; a peeled, milk-only mash loses about 10%. The honest statement is that a pound of raw potatoes makes 14 to 19 oz of mash, and a number quoted without its unit is not enough to shop with.
The disagreement that actually matters
Once the units are fixed, a real spread is left, and it is about portion size rather than arithmetic. Professional kitchens serve mash in a No. 8 scoop — half a cup. Thanksgiving hosts serve it in a mound.
| Setting | Mash per person | Raw potatoes |
|---|---|---|
| School / foodservice scoop (USDA) | ½ cup · 4½ oz | 3.8 oz (110 g) |
| Idaho Potato Commission, 25 people | ½ cup · 4½ oz | 3.8 oz (6 lb / 25) |
| Everyday dinner side | ⅔ cup · 5–6 oz | 1/3 lb (150 g) |
| Thanksgiving, mash is a headline dish | 1 cup · 7–8 oz | 1/2 lb (225 g) |
| Leftovers wanted, or a mash bar | 1½–2 cups | 3/4–1 lb (340–450 g) |
The institutional figures agree closely — the USDA recipe and the Idaho Potato Commission independently land on 3.8 oz of raw potato per head — and they are roughly half the holiday figure. Both are right: one is a side dish on a tray of five, the other is the thing people go back for. If your table also carries stuffing, sweet potatoes and a green bean casserole, drop to 1/3 lb. If mash and turkey carry the plate, stay at 1/2 lb.
What to buy, by guest count
At 1/2 lb of raw potatoes per person, in the 5 lb bags supermarkets actually sell:
| Guests | Raw potatoes | What to buy |
|---|---|---|
| 8 | 4 lb | One 5 lb bag |
| 10 | 5 lb | One 5 lb bag |
| 15 | 7½ lb | Two 5 lb bags |
| 20 | 10 lb | Two 5 lb bags |
| 30 | 15 lb | Three 5 lb bags |
The full range, including the lighter and heavier portions:
| Guests | Light (1/3 lb) | Standard (1/2 lb) | Leftovers (3/4 lb) |
|---|---|---|---|
| 4 | 1½ lb | 2 lb | 3 lb |
| 6 | 2 lb | 3 lb | 4½ lb |
| 10 | 3½ lb | 5 lb | 7½ lb |
| 12 | 4 lb | 6 lb | 9 lb |
| 20 | 6½ lb | 10 lb | 15 lb |
| 25 | 8½ lb | 12½ lb | 19 lb |
| 40 | 13½ lb | 20 lb | 30 lb |
| 50 | 17 lb | 25 lb | 37½ lb |
Round up to whole bags: raw potatoes keep for weeks in a cool dark cupboard, whereas leftover mash keeps three days and reheats badly without extra cream.
If you are shopping in metric
Ireland's state food safety body, safefood, tells Christmas hosts to "plan for about 2 medium sized potatoes per person, or 250 grams." That is 8.8 oz — slightly above the American half-pound, and notable because safefood exists to cut food waste. When a waste-reduction agency and a US food magazine agree within an ounce, the figure is about as solid as portion advice gets. Their 250 g is for roast potatoes, where nothing is added back.
The honest caveats
Two things this page cannot tell you. First, variety changes the yield — a floury russet absorbs more butter and cream than a waxy Yukon Gold, and no source we found quantifies it. Second, the 20% peeling figure rests on a single source. The Idaho Potato Commission is a state commodity commission rather than a marketing blog, which makes it the best number available, but we found nothing independent to check it against. Peel thinly and you will beat it; take the skin off with a knife and you will not.
The rest of the plate runs on the same logic: how much gravy per person covers the one dish capped by an input you cannot scale, and Christmas dinner quantities per person sizes the whole table at once.
Sizing the rest of the meal? Start with how much turkey per person — everything else on the table scales against it, and it has the same raw-versus-cooked trap built in.