Check a supplier price increase line by line

Ask for the full new price list, not just the letter. Compare every line per unit with the list you pay today, then weight each change by the quantity you actually buy. The supplier's "average 5 %" is usually a plain average of percentages; what it costs you can be quite different — and the few lines that make the difference are the ones to negotiate.

Price lists · 29 September 2026 · Proceny

An example: "about 5 %"

A distributor writes that prices rise by an average of 5 % from the 1st of next month. You buy six of its items. The quantities are last year's purchases from your ERP.

The announced increase on six items
Item	Old	New	Change	Yearly qty	Old spend	Extra cost
Cable gland M20	0.84	0.88	+4.8 %	12,000	10,080	480
Junction box IP65	6.40	6.72	+5.0 %	1,500	9,600	480
Terminal block 4 mm²	0.62	0.65	+4.8 %	20,000	12,400	600
Circuit breaker 16 A	4.10	4.80	+17.1 %	3,000	12,300	2,100
Cable ties, bag of 100	3.20	3.20	0.0 %	800	2,560	0
LED panel 600 × 600	28.00	27.20	−2.9 %	200	5,600	−160
Total					52,540	3,500

Three ways to summarise the same six lines
Measure	Result	What it says
Average of the six percentages	+4.8 %	The supplier's "about 5 %"
Median (middle) change	+4.8 %	The typical line
Weighted by your spend	+6.7 %	What the increase costs you: 3,500 on 52,540

The announcement is accurate and still misleading for you: the biggest increase sits on your biggest line. The circuit breakers are 23 % of your spend with this supplier and 60 % of the extra cost. That line, not the "5 %", is the conversation to have.

The formulas

With old unit prices in B, new in C and your yearly quantities in E (rows 2 to 7):

Change per line        =C2/B2-1
Extra cost per line    =(C2-B2)*E2
Weighted increase      =SUMPRODUCT((C2:C7-B2:B7)*E2:E7)/SUMPRODUCT(B2:B7*E2:E7)
Typical change         =MEDIAN(D2:D7)
Lines above the claim  =FILTER(A2:A7, D2:D7>5%)

Sort by extra cost, largest first. On a real list a handful of lines usually carry most of the money; those are the ones worth an e-mail.

Before you trust any percentage: per unit

A price increase is often mixed with other changes that make the raw percentage wrong. Check these first, or your analysis starts from the wrong numbers:

Pack size. A box of 10 at 24.00 that becomes a box of 12 at 27.60 looks like +15 %. Per piece it is 2.40 → 2.30, a decrease of 4.2 %.
Price basis. A price per 100 read as a price per piece shows up as +9,900 % — or hides a real increase if both lists changed basis. See price per 100 and pack size changes.
Renumbered items. A code that disappears and a new code that appears may be the same product at a new price. Pair them by manufacturer part number before you count one as dropped.
Currency. A list re-issued in another currency is not an increase until you fix the rate you will pay.

Pairing two lists row by row, safely, is covered step by step in how to compare supplier price lists.

What else to look at

Items that disappeared. A dropped line you still buy means a substitute, usually at a different price. Ask what replaces it.
Minimums and breaks. A higher minimum order or a moved price break is an increase that no price column shows.
Lines that went down. Keep them in the total. An analysis that counts only increases overstates the change, and the supplier will point it out.
Quantities. Last year's purchases are a fair start; use the forecast if volumes are changing.

What to ask the supplier

The new list with item codes, units, pack sizes and the price basis — as a spreadsheet, not only a PDF.
The date the prices apply from, and whether open orders and confirmed quotes keep the old prices.
For the lines well above the announced figure: what changed? A raw-material or freight cost is a reason you can check against a public index; "market conditions" is not.
Whether your volume on the largest lines earns a smaller increase, or a later date — with your numbers from the table above.