The division is trivial; the money is in what happens when the racks a tenant installs are not the density the hall was designed for.
Divide 1,000 kW by the design power per rack. At illustrative densities, one megawatt of critical IT supports 125 racks at 8 kW, 66.7 at 15 kW, 25 at 40 kW and about 7.7 at 130 kW. The arithmetic is trivial; the commercial point is that a hall laid out for one density and filled at another strands either power or floor, and on an illustrative 10 MW hall that can cost $4.2m a year of rent.
Worked in full in The Data Center Development Handbook by Julian R. Sterling, with every figure reproduced in a free workbook.See the book on Amazon →
Developers sell megawatts, tenants buy racks, and the translation between the two is where design briefs go wrong. Racks per MW decides the floor plate, the cooling plant and how much of the building a tenant can actually use, so it belongs in the underwriting from the first site test, not in the fit-out.
| Input | Value |
|---|---|
| Critical IT capacity | 1 MW = 1,000 kW |
| White space per rack position, aisles included | 30 sq ft |
| Data hall size | 10 MW |
| Design density of the hall | 15 kW per rack |
| Rent (illustrative turnkey) | $105 per kW-month |
Critical IT capacity is the power available to the racks after redundancy and before cooling overhead. The grid connection is larger by the peak PUE, which is a separate calculation.
Racks per MW = 1,000 ÷ kW per rack
White space per MW = racks per MW × sq ft per rack position
Rent per rack per month = kW per rack × rent per kW-month
In Excel: =1000/kW_per_rack and =ROUNDDOWN(Hall_kW/kW_per_rack,0) for whole positions in a hall. Keep the rent in kW: a rack price is an output, not an input.
| Rack type | kW per rack | Racks per MW | Racks in 10 MW | Sq ft per MW | W per sq ft | Rent per rack-month |
|---|---|---|---|---|---|---|
| Enterprise, air-cooled | 8 | 125.0 | 1,250 | 3,750 | 267 | $840 |
| High-density, air-cooled | 15 | 66.7 | 667 | 2,000 | 500 | $1,575 |
| AI inference, liquid assist | 40 | 25.0 | 250 | 750 | 1,333 | $4,200 |
| AI training, direct liquid | 130 | 7.7 | 77 | 231 | 4,333 | $13,650 |
Rent per megawatt is identical down the table: $105 per kW-month whatever the rack. What changes is how much floor, and what kind of cooling, it takes to deliver that megawatt. A liquid-cooled training hall delivers the same MW on 231 sq ft of white space against 3,750 for enterprise racks, which is why per square foot comparisons between them are meaningless; the conversion is in data centre rent per kW against per square foot.
Take the 10 MW hall laid out for 15 kW racks: 10,000 ÷ 15 = 666.7, so 667 positions, with cooling sized for 15 kW at each. A tenant arrives with 10 kW racks.
The mismatch the other way is no better. A tenant wanting 25 kW racks needs only 400 positions to reach 10 MW, leaving 267 empty, but the cooling serves 15 kW per position, so the denser racks cannot be installed without rework. Power is stranded in the first case, floor and cooling in the second.
| Actual kW per rack | MW used | MW stranded | Rent lost a year |
|---|---|---|---|
| 8 | 5.33 | 4.67 | $5.9m |
| 10 | 6.67 | 3.33 | $4.2m |
| 12 | 8.00 | 2.00 | $2.5m |
| 15 | 10.00 | 0.00 | $0.0m |
Who bears it depends on the lease. If the tenant has contracted the full 10 MW, the stranding is its problem and the rent is still paid. If the hall is leased by the rack or in phases, the landlord carries it. The density assumption is therefore a lease term as much as a design one.
The common mistake is to design to an average density. A hall averaging 15 kW per rack across a mix of 8 kW storage and 40 kW compute rows will strand capacity in both kinds of row unless the layout and the cooling are zoned to match. The second mistake is to quote rack counts from the grid connection rather than from critical IT: a 10 MW connection at a 1.40 peak PUE carries about 7.1 MW of IT, so 476 racks at 15 kW, not 667. That relationship is worked in how a higher PUE limits leasable capacity.
Racks per MW is 1,000 divided by kW per rack: 125 at 8 kW, 7.7 at 130 kW. The number that matters to a developer is the one beside it, how much of the megawatts a given tenant's racks can actually use, here 6.67 of 10 MW and $4.2m a year if the density is wrong. The free workbook for this book carries the campus underwrite the rent and capacity figures feed into.
It varies by generation and cooling, so treat any figure as illustrative. Enterprise air-cooled halls are often designed around single-digit to mid-teen kW per rack; liquid-cooled AI training racks can exceed 100 kW. At 8 kW a MW of IT supports 125 racks; at 130 kW it supports 7.7, so the same megawatt needs a fraction of the floor and a very different cooling plant.
Racks per MW times the floor each rack position takes, aisles included. With an illustrative 30 sq ft per position, 1 MW needs 3,750 sq ft at 8 kW per rack, 2,000 at 15 kW and about 231 at 130 kW. Mechanical and electrical plant rooms come on top and grow with the megawatts, not the racks.
Power or cooling that is built and paid for but cannot be sold, because another constraint ran out first. In an illustrative 10 MW hall with 667 positions designed for 15 kW, 10 kW racks fill every position at 6.67 MW, stranding 3.33 MW, or $4.2m a year of rent at $105 per kW-month.
This article is one calculation from The Data Center Development Handbook. The book takes the same case from first principles to the decision, chapter by chapter, and every figure it prints is a live formula in the free companion workbooks.
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