The Navi Mumbai campus is 5.5 acres — 2.2 hectares — and it holds 8.8 MW of IT capacity today across MUM-1 and MUM-2, which is 1.6 MW per acre. MUM-3 adds 85 MW at full build, taking the campus to 93.8 MW and 17.1 MW per acre. The same piece of land, 10.6 times the megawatts. That is not a marketing artefact — the operator has approved a 120 MVA substation, and 120 MVA divided by 85 MW is 1.41, which is exactly a design PUE of about 1.4 with no headroom. The substation is the proof that the 85 MW figure is an engineering number rather than a headline.
1At a glance
2Ten times the megawatts on the same 5.5 acres
Land efficiency is the cleanest comparison in Indian data centres because land is the one input nobody can add. Every operator in Mumbai is buying the same scarce parcels, and the only way to put more megawatts on a fixed footprint is to build upward and to push power density per square metre.
| Stage | MW | MW per acre | What it means |
|---|---|---|---|
| MUM-1 alone, 2015 | 2.5 | 0.45 | Single-storey era, enterprise colocation |
| MUM-1 + MUM-2, 2023 | 8.8 | 1.6 | Two buildings, same parcel, no new land |
| MUM-3 at full build | 85 | 15.5 | One building, 9.7 times the output of the first two |
| Campus at completion | 93.8 | 17.1 | Densest campus in the registry |
| For comparison — BOM3 Chandivali | 48 | 12.9 | Densest announced single facility in the registry |
| For comparison — Kolkata 1 | 25 | 3.3 | Low-rise, land-rich, 7.5 acres |
MUM-1 at 2.5 MW on 5.5 acres is 0.45 MW per acre — a 2015 building on a site bought for expansion that had not happened yet. MUM-2 doubled the output without buying land.
MUM-3 puts 85 MW on the remainder. The 5.5 acres does not change. What changes is storeys, cooling architecture and the substation behind the wall.
This is the single most useful ratio for anyone pricing Indian land: the site is fixed and the megawatts are a design decision. AirTrunk pays ₹118 crore an acre in Chandivali because 12.9 MW per acre is the only way the land cost works.
3120 MVA for 85 MW
Almost every capacity claim on this registry is unverifiable from the outside. MUM-3 is one of the rare exceptions, because the power infrastructure is published and it has to be sized to the load.
| Ratio | Value | Reading |
|---|---|---|
| MUM-3 IT capacity | 85 MW | The headline claim at full build |
| Dedicated substation | 120 MVA | Approved at the same time as the 2023 announcement |
| Substation ÷ IT load | 1.41 | The implied facility-to-IT ratio |
| Declared design PUE, AirTrunk comparable | below 1.4 | The Indian hyperscale norm in 2026 |
| At 1.41 with no headroom | zero spare | The substation is sized to the claim, not above it |
| If the load were 32 MW as announced in 2023 | 3.75 | Three and a half times oversized — implausible |
The arithmetic cuts both ways and both readings are useful. A 120 MVA substation for 85 MW of IT load is a facility-to-IT ratio of 1.41, which is precisely what a modern Indian hyperscale build runs at. That makes the 85 MW credible.
But when the same 120 MVA was announced alongside a 32 MW building in June 2023, the implied ratio was 3.75 — nobody builds a substation three and a half times oversized. Which means the substation was sized for a bigger building than the one announced, and the 85 MW figure was always the real plan.
This is the method that makes capacity data defensible. Do not trust the megawatt number; look for the substation, the transformer capacity or the power purchase agreement and divide. A ratio between about 1.35 and 1.5 validates the claim. A ratio of 2 or 3 means the claim is low, and a ratio below 1.2 means the claim is inflated.
Applied across this registry, it separates the 85 MW that will get built from the 3 GW that is a letter of intent.
432 MW to 85 MW in thirty months
- MUM-3 announced at 32 MW IT load, Tier III designed, on a five-acre parcel. Investment US$170 million. Go-live H2 2025. A 120 MVA substation is approved with the announcement.
- The original go-live passes with no commissioning announcement.
- Iron Mountain takes 100% of Web Werks. MUM-3 becomes a wholly owned build rather than a joint venture asset.
- Ground broken. The facility is now published as 85 MW hyperscale-ready and AI-oriented, 130,000 sq m, with Stirling & Wilson building it.
- Operator pages split between 2027 delivery and Q1 2028 completion. No commissioning date, no phasing plan, no capex for the 85 MW configuration published.
| Item | June 2023 | January 2026 | Change |
|---|---|---|---|
| IT capacity | 32 MW | 85 MW | 2.66× |
| Go-live | H2 2025 | 2027, or Q1 2028 | +2 to 2.5 years |
| Architecture | Tier III, traditional raised floor | AI-ready, high-density, specialised cooling | Different product |
| Substation | 120 MVA | 120 MVA | Unchanged |
| Capex | US$170 million | Not published | Unknown |
The unchanged substation is the tell. A 120 MVA connection is what a 85 MW building needs. It was never what a 32 MW building needed. The published capacity went up to meet the infrastructure that had already been approved.
What is not published is capital cost. At the US$170 million for 32 MW rate the implied cost was US$5.31 million per MW. Applying that same rate to 85 MW gives US$451 million, and applying AirTrunk’s published programme rate of US$6.0 million per MW gives US$510 million — about ₹4,510 crore. Both are Gridvik estimates, not disclosed figures, and both are labelled as such.
5Procurement
6Limits and open questions
- 93.8 MW is a completion sum, not a plan. The operator states 90+ MW on one page and 93 MW on another; 93 uses 8.3 MW for MUM-1 and MUM-2 while the location pages now sum to 8.8 MW.
- MUM-3 delivers in 2027 or Q1 2028 depending on which operator page is read. Both are live.
- The 120 MVA inference assumes a power factor of 1.0. At 0.95 the implied ratio falls to 1.34 and the claim looks conservative rather than exact.
- No capex is published for the 85 MW configuration. Every per-megawatt cost on this page is a Gridvik estimate at a stated rate.
- The campus ratio of 17.1 MW per acre is not a facility ratio. It mixes two operating buildings with one unbuilt one; the registry keeps BOM3 Chandivali as the densest single announced facility at 12.9 MW per acre.
- MUM-3 may sit on a separate five-acre parcel. The 2023 announcement describes a five-acre parcel acquired for the third building, separate from the 5.5-acre MUM-1 and MUM-2 campus. If so the combined site is about 10.5 acres and the completion ratio falls to 8.9 MW per acre, behind Chandivali and BOM1. Unresolved, and the reason this registry does not claim the campus record.
- Zero megawatts counted. MUM-1 and MUM-2 count as zero in the source file; MUM-3 is flagged non-additive.
- What would close it. A phase schedule with megawatts attached to dates.
7Sources
Sources: Iron Mountain Mumbai and MUM-1, MUM-2 location pages; the Iron Mountain India platform page; the January 2026 MUM-3 groundbreaking release; Data Center Dynamics coverage of the groundbreaking and of the 2023 joint venture announcement; W.media; ETTelecom; the Baxtel MUM-3 entry; the Web Werks and Iron Mountain June 2023 announcement; and SiliconIndia on the five-acre parcel acquisition.