Parties, hosting and get-togethers

How much to buy, what size tin to bake in, when everything goes in the oven and who owes what afterwards. Set the number of guests once at the top of the note, and if four more say yes on the Thursday, change that one number and the whole shopping list moves with it.

How much to buy

Everything at a party is “some amount per person”. Write it that way and the list works itself out.

calcCopy

# Drinks party for 22

guests = 22 gives 22

# Three glasses each, 175 mL a glass

wine = guests * 3 * 175 mL in L gives 11.55 L

bottles = ceil(wine / 750 mL) gives 16

# Half of them will want fizz to start

fizz = ceil(guests / 2 * 125 mL / 750 mL) gives 2

beer = guests * 2 gives 44

soft drinks = guests * 500 mL in L gives 11 L

ice = guests * 250 g in kg gives 5.5 kg

glasses = guests * 1.5 rounded up gives 33

ceil rounds up, because the shop won’t sell you four-fifths of a bottle. Change guests to 30 and every line follows, which is the whole trick: you write the party once and then just edit the number at the top. See Rounding up and down.

Food works the same way, from whatever per-person figure you trust.

calcCopy

guests = 22 gives 22

canapés = guests * 6 gives 132

rice = guests * 75 g in kg gives 1.65 kg

salad = guests * 120 g in kg gives 2.64 kg

cheese = guests * 70 g in kg gives 1.54 kg

loaves = ceil(guests / 8) gives 3

tables = ceil(guests / 8) gives 3

# You have fourteen chairs

guests - 14 gives 8

The last line is the one that saves the evening: eight chairs to borrow, worked out on the Tuesday rather than at seven o’clock on the Saturday.

A tin that isn’t the one in the recipe

The recipe wants a 20 cm round tin and yours is 23 cm. Compare the two areas and you have the number to multiply every ingredient by.

calcCopy

# Recipe tin against the one in the cupboard

recipe tin = pi * (10 cm)^2 gives 314.159265359 cm²

my tin = pi * (11.5 cm)^2 gives 415.4756284373 cm²

scale = my tin / recipe tin gives 1.3225

200 g * scale gives 264.5 g

3 eggs * scale rounded up gives 4

# A square tin instead

20 cm by 20 cm gives 400 cm²

400 cm² / recipe tin gives 1.2732395447

A tin’s area is what decides how deep the mixture sits, so the ratio of the areas is the ratio to scale by. A square 20 cm tin holds about a quarter more than a round 20 cm one, which is why the same recipe comes out flatter in it. Eggs round up, since half an egg helps nobody. See Squares, cubes and powers.

One oven, one clock

Work backwards from when you want to eat. Each line is a countdown to the same moment, so you can read the whole afternoon off the block.

calcCopy

# Sunday lunch for eight, eating at 2pm

eat at = 2:00pm gives 2:00 PM

beef = 2.2 kg gives 2.2 kg

roasting = beef * 25 min/kg + 20 min gives 1 hour 15 min

resting = 20 min gives 20 min

Beef in: eat at - roasting - resting gives 12:25 PM

Potatoes in: eat at - 50 min gives 1:10 PM

Yorkshires in: eat at - 25 min gives 1:35 PM

Veg on: eat at - 12 min gives 1:48 PM

Oven on to heat: eat at - roasting - resting - 15 min gives 12:10 PM

25 min/kg is a rate, so multiplying it by the joint gives the roasting time. Writing a label before the colon turns the block into a running order you can read down while you cook, and the labels stay out of the arithmetic. See Labels.

What it came to, and who owes what

Add the receipts up as they land, then share it out.

calcCopy

# Saturday

drink = £84.60 gives £84.60

food = £126.40 gives £126.40

cake = £22 gives £22.00

flowers = £18.50 gives £18.50

party total = total gives £251.50

# Eight of us chipped in, one couldn't make it

split party total among 7 gives £35.93

party total / 7 rounded up to nearest £1 gives £36.00

# A joint present, nine people

£140 / 9 rounded up to nearest £1 gives £16.00

A line that says only total adds up the lines above it, so you never have to name each receipt, and party total = total gives that sum a name so the lines below can go on using it. Rounding each share up to the pound means the pot covers the bill with a little over, which is kinder than asking one person for the 43p. See Adding up the lines above and Rounding to a step.

Counting down

Keep the date at the top of the note and let it tell you where you are.

calcAnswers depend on today’s date or live dataCopy

party = 12 December 2026 gives December 12

weekday on party gives Saturday

days until party gives 85 days

whole weeks until party gives 12 weeks

# Send the invitations six weeks ahead

party - 6 weeks gives October 31

# Order the cake a fortnight before

party - 1 fortnight gives November 28

These count from today, so the note tells you something new each time you open it. weekday on checks you’ve picked the Saturday you thought you had. See Counting down.

Make it your own

• Feeding people outdoors: 8 burgers at £1.35 each and 3 packs of buns @ £1.10 price a barbecue line by line, with total underneath.
• Scale a punch recipe to the bowl you own: 4 L / 200 mL gives 20 glasses, and serves 6 scaled to 15 is just 15 / 6 as a number to multiply by.
• Work out a per-head budget: with budget = £400 and guests = 22, budget / guests gives £18.18 a head.
• Check the drinks order against the offer: £9.50 x 6 with 25% off gives £42.75 for a mixed case.
• Chill things in time: 6:00pm - 3 hours is when the white wine goes in the fridge.
• Cater for a school cake sale: 36 cupcakes at 90p gives £32.40 if it all sells.
• Split a holiday house by nights: with house = £1,800 over nights = 5, house / nights gives £360.00 a night, and house / nights / 6 people gives each person’s share.
• Count the sleeps for whoever is most excited: sleeps until 25 December 2026.
• Plan a picnic in the sun: salad = 22 * 120 g in kg scales the same way as everything else here.
• Share a taxi home: £38 between 4 of us gives £9.50 each.
• Work out how long the playlist needs to be: 6:00pm to 11:30pm is 5 hours 30 min of music.