A goods-supply price schedule with one index series circled, read before the clause is signed.

CPI vs RPI: How to Read a Price-Indexation Formula, Cap, and Floor

Name one published series, which charges move, and whether the cap and floor hit this year's change or the original base, then narrow the clause or walk.

October 15, 2026•10 min read•Article

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Key takeaway in 30 seconds

A goods-supply indexation clause names one published series, the charges that move, and whether the cap and the floor hit this year's percentage or the original unit price. CPI and RPI are not one basket: owner-occupiers' housing costs and Council Tax sit in RPI and CPIH, not in CPI. A cap plus a floor is not a band that always moves the price. Write a six-line gap log, then narrow the silent formula or walk.

Before you sign a goods-supply price schedule, name one published series and say which base the cap and the floor hit, then narrow the clause or walk. CPI and RPI are not one basket, and a cap plus a floor is not a corridor that always moves the price. The contract is governed by the law of England and Wales.

Test three claims — same basket, already printed, and silence accepts — against the clause, not the email.

Hefin at Dyfi Board Ltd has heads of 18 September 2026 for ash blanks from Wye Timber Supplies Ltd: ONS CPI all items, D7BT (2015=100), AB-40 at £6.40. Clause 9.2 takes the higher of CPI and RPI, cap 4%, floor 1%, and clause 9.3 lets the supplier pick the substitute. The 8 October 2026 email says the indices match, September is out, and silence accepts. Friday 16 October 2026 is the decision meeting, not a review date.

Disclaimer: Checkory provides AI support, not legal advice. Consult a qualified lawyer for binding decisions.

Which published series does the clause actually name?

Name the publisher, the identifier, the geography, and whether housing sits in the basket. If a line is blank, stop. Two names are not one series, and taking the higher is not the D7BT the heads agreed.

The heads name D7BT, CPI all items, 2015=100, UK. RPI all items is CHAW, January 1987=100. Those levels do not form an inflation rate. Clause 9 writes neither code. A typical mistake is to treat those two codes as one rate.

Housing costs and Council Tax sit in RPI and CPIH, not in CPI. RPI uses Carli and Dutot; CPI and CPIH mainly use Jevons. From February 2006 to December 2018 that effect averaged about 0.6 percentage points, and not less than 0.3, on the CPIH-RPI 12-month gap (ONS methodology).

RPI is no longer accredited official statistics. Guidance of 25 March 2026 says John Pullinger would have strongly discouraged its use, and that CPIH is the ONS lead measure.

CPI all items, D7BT

Pros

✓Heads: UK, 2015=100.
✓Housing costs and Council Tax sit outside.

Cons

✗Clause 9 has no code.
✗The word CPI alone is not D7BT.

RPI all items, CHAW

Pros

✓January 1987=100. Never revised once published.
✓Housing sits inside.

Cons

✗Not the series in the heads.
✗February 2030 is a method shift, not a deletion today.

Two index cards in the published-series section, with a housing-cost tick only on the retail-prices card. — Housing costs sit in RPI and in CPIH. They do not sit in CPI. The heads named D7BT only.

Which charges is the formula allowed to move?

Mark only the charges the text names. A fixed line stays fixed until a sentence points at it. The Price, with no schedule reference, does not move every figure on the invoice.

The heads move only the Schedule 1 unit price. The £480 setup, tooling, and carriage stay fixed. Clause 9 does not point at Schedule 1. For example, 4% is not an uplift on the setup fee until the clause names it. TLT (January 2023) keeps a one-off fixed. Zephior (5 September 2026) separates that sum before the factor.

What if the reference month is not the publication day?

Reference month, publication day, notice, and revision stay separate. A missing figure is unpublished. Silence is not a print, and it is not acceptance.

Each 1 December, divide the index three months earlier by the index three months before the previous 1 December. First comparison: three months before 1 December 2026, on the preceding year price. Notice is 20 Business Days, or 10 after a late print, then the next invoice. The term is three years from 1 December 2026. Clause 9.4 treats silence as acceptance.

In practice the bulletin is at 7:00, often between the 13th and the 21st (ONS). On 15 October 2026 the pages showed a release on 16 September 2026 and a next release on 21 October 2026. September was not verified as printed. Published RPI is never revised; CPI and CPIH move only in the public interest. Do not type an August or September 2026 level.

Four date marks on a supply price schedule in the reference-month section, separating the reference month, the print, the notice, and the next invoice. — Pages checked on 15 October 2026 showed a next release on 21 October 2026. That is not a September print in clause 9.

Do the cap and the floor hit this year's change or the original price?

Read threshold, cap, floor, rounding, then the base, in the written order. The preceding year price and the original unit price are not the same base. If the draft picks neither, mark the gap. Do not calculate £6.40.

The factor is the comparison index divided by the base index. Percentage equals (factor minus 1) times 100. Index points are not a percentage. Zephior says the written order is the method. Its 200-to-220 example is not AB-40.

Heads apply 5% and 0% to that year percentage before the preceding year price. A fall does not cut the price. Clause 9.2 is 4% and 1% with no base. Do read the cap and the floor in the written order, and name the base. Don't treat a 4% cap and a 1% floor as a corridor that always moves the price.

TLT prefers the original price so old rises stay out. Its 1% and 5% examples are not clause 9. Travers Smith: growth since the last review is not one percentage on the review date. A collar means a minimum floor.

Arnold v Britton is a lease step of ten pounds per hundred (press summary), so do not import 10% or £90. No express right means no solo rise (CMS). Sections 8(2) and 15(1) do not fill a later-agreed price (Norton Rose Fulbright), and KSY kept an open price (Gowling WLG). Neither note chooses D7BT.

Can the price fall, and who names a substitute series?

One-way uplift does not let the price fall. Split discontinuation, a method change, and a late print, and name who picks the substitute.

Clause 9.1 says increased. Heads use a 0% floor, so a fall in the index does not cut the price. An undefined 1% floor may force a rise. Do not calculate the fall on a blank base.

On 15 October 2026 RPI was still published. February 2030 is earliest; the Chancellor had not agreed sooner. Lower-level RPI indices then cease. 5 September 2025 minutes say the reform is still planned. They do not say that no final decision was taken.

Clause 9.3 lets the supplier choose if an index is unavailable. The heads name L522 if D7BT stops or its methods change, then an ICAEW President expert who must pick a published UK household index and must not invent a number. Do not treat an L522 next-release date as current.

The FIT licence from 31 March 2026 swaps RPI for CPI on the previous year tariff (licence text). The 1 April 2026 table used 3.4 percent. That rate is not AB-40.

A six-item checklist for the gap log before you sign, with the fallback row circled, beside a blank details notebook. — An empty row is a gap between the heads and clause 9. It is not a figure to type onto AB-40.

What belongs on the gap log before you sign?

Set the heads beside clause 9 and write six lines before you sign. An empty row is a gap, not a number you may invent. Narrow the silent rows toward the heads, or walk.

Read it as series → charges → base month and review month → which base the cap and the floor hit → one-way or a fall → fallback.

Heads against clause 9

Question	Heads	Clause 9	What to do
Series	D7BT, 2015=100, UK	Higher of CPI and RPI	Name one series
Housing	Outside CPI	Not said	Keep the baskets apart
Charges	AB-40 unit price	The Price	Hold £480, tooling, carriage
Base	Preceding year price	Silent	Write one base
Cap and floor	5% and 0% on this year, first	4% and 1%, no order	Write the order. Leave £6.40
A fall	Price does not fall	Increased, 1% undefined	Not a 1 to 4 band
Late print	Next invoice after notice	Silence accepts	8 October email is not a print
Fallback	L522, then ICAEW expert	Supplier chooses	Name the method change
Revision	Not said	Not said	RPI is never revised. Record the CPI vintage

Checklist before the price schedule is signed

Name the series

Heads say D7BT. Clause 9 has no code and takes the higher of.

Mark the charges

Unit price only. £480, tooling, and carriage stay fixed.

Split the dates

Three months before 1 December. 20 Business Days, or 10 after a late print.

Name the cap base

5% and 0% before the preceding price. Do not calculate £6.40.

Mark the fallback

Heads name L522, then an ICAEW expert.

Narrow or walk

An empty row means edit or walk, before Friday 16 October 2026.

End on the log. Narrow clause 9 toward the heads, or walk before Friday. A lease rent review (rent review), a SaaS yearly percentage (annual uplift), and a most-favoured-customer promise (price parity) are different papers. So are a green-lease cost split and a construction fluctuation formula.

After the log, not instead of it, run a first-pass — a first read that flags gaps, not a lawyer sign-off — through document analysis.