What's behind FDDI v2.0
FDDI is the cornerstone index of the Infralytics-GIIS platform. It looks simple — Full Fibre premises ÷ total premises — but the v2.0 register adds tier thresholds, formula versioning, and Ofcom-authoritative premises denominators. Here's the full mechanics.
FDDI — Full Fibre Density Index — is the headline metric on Infralytics-GIIS. Every Identity Page, the Highlight Reel podium, the Compare table, and the Critical Gap detection logic anchor on it. This post explains the v2.0 mechanics.
The formula
FDDI = full_fibre_premises ÷ local_authority_total_premises
It looks deceptively simple. Two design choices distinguish it from "% Full Fibre availability" headline figures you'll see in press releases:
- Premises denominator is Ofcom-authoritative. We store
local_authority_total_premisesseparately from any derived count and refresh it on every Connected Nations vintage. County Durham, for example, sits at 251,998 premises; we never round, never approximate, and never use ONS estimates as a substitute. - The numerator is residential + business premises with FTTP availability, not take-up. Take-up is tracked separately as
ff_takeup_coverage.
The v2.0 Tier Threshold Register
| Tier | FDDI band | What it signals |
|---|---|---|
| Leader | ≥ 0.90 | Top-decile rollout; further build = diminishing returns |
| Mature | 0.70 – 0.90 | Sustained build; gaps now in deprivation-correlated areas |
| Developing | 0.40 – 0.70 | Active rollout window; biggest two-year deltas typically land here |
| Critical Gap | < 0.40 | Below 4-in-10 premises ready; civils + permits the binding constraint |
These thresholds were set against the March 2026 v2.0 register and apply uniformly across all authorities. They are not regionally normalised.
What FDDI feeds
FDDI is not an end in itself — it's an input to four downstream indices:
- CEI v2.0 (Copper Exposure Index) · 1 − FDDI. Direct algebraic inverse — high FDDI ⇒ low copper exposure.
- IRS (Infrastructure Resilience Score) · 0.5·FDDI + 0.5·(Gigabit/100). Composite resilience.
- DDCI (Deprivation-Digital Coverage Index) · derived from FDDI delta between top and bottom IMD quintiles.
- DERS (Digital Equity & Resilience Score) · 0.30·CEI + 0.30·(1−HSP) + 0.20·UC + 0.20·(1−skills). Currently 0.50 complete pending UC + digital-skills data.
The chain matters: a councillor asking "how do we improve DERS?" is, in part, asking "how do we improve FDDI?"
What changed from v1.1 → v2.0
- v1.1 used derived premises counts (rolled up from postcode-level coverage) as the denominator. v2.0 switched to Ofcom-authoritative LA-level premises to remove postcode-aggregation drift.
- v2.0 introduced formula version metadata on every index. Every API response carries
formula_versionso downstream consumers can guard against silent index re-definitions. - Tier thresholds were codified rather than embedded in chart styling.
What's next
The November 2026 vintage will be the first full FDDI v2.0 dataset against the new premises basis. Two LAs (South Tyneside, Gateshead) are the leading candidates to cross into the Mature band.
See the live tier register →
/api/tier-registry. Inspect the formula in the Analyst console →/analyst/dashboard.