Flyvbjerg Project Database: The Iron Law Source
Bent Flyvbjerg's Oxford project database is the most-cited empirical source on big-project performance. We give the sample, methodology and citation URL.
What is the Bent Flyvbjerg database?
The Bent Flyvbjerg project database is the largest dataset of its kind in the world: more than 16,000 large projects, assembled over decades by Flyvbjerg and his team at Oxford's Saïd Business School and still expanding. For each project it records the budget at the decision to build against the final outcome, across rail, road, tunnels, dams, energy, IT, defense, and the Olympic Games. It is the empirical basis for Flyvbjerg's Iron Law of Megaprojects and for reference class forecasting. Only 0.5% of the projects in it come in on budget, on time, and with the promised benefits.
Average overrun by category (Flyvbjerg-Gardner 2023)
| Project category | Mean cost overrun | % of projects in the fat tail (50%+ overrun) | Mean overrun of that tail |
|---|---|---|---|
| Solar power | +1% | 2% | +50% |
| Energy transmission | +8% | 4% | +166% |
| Wind power | +13% | 7% | +97% |
| Tunnels | +37% | 28% | +103% |
| Rail | +39% | 28% | +116% |
| Airports | +39% | 43% | +88% |
| Buildings | +62% | 39% | +206% |
| IT | +73% | 18% | +447% |
| Nuclear power | +120% | 55% | +204% |
| Olympic Games | +157% | 76% | +200% |
| Nuclear waste storage | +238% | 48% | +427% |
From the base-rate appendix of Flyvbjerg and Gardner (2023), How Big Things Get Done, Penguin Random House. Overruns are real (inflation-adjusted) against the budget at the decision to build. The IT fat tail is the source of the widely-quoted 447% figure: the 18% of IT projects that go more than 50% over budget overrun by 447% on average.
What counts as a megaproject
Flyvbjerg defines a megaproject as a project with a budget of 1 billion US dollars or more. The dataset also includes large projects below this threshold for categories where 1bn is unusual (some IT projects, smaller renewables). Olympic Games, defense procurement above the relevant defense-major threshold, and nuclear plants are always included.
The unit of analysis is the project, not the contract. Cost overrun is measured in real terms (inflation-adjusted) against the originally approved budget at the decision-to-build stage, not against later re-baselined figures. This is critical: many megaproject business cases re-baseline the budget partway through, which would otherwise mask the overrun.
Can you access or download the database?
Short answer: not the raw data. The full database is a proprietary research asset, assembled and curated over decades by Flyvbjerg and his team (originally at Oxford's Saïd Business School), and it is not published as a downloadable file. There is no public spreadsheet, API, or open data portal for the 16,000-plus project records themselves.
What is public is the analysis built on it, and that is what you cite:
- The category base rates (the table above) are published in the appendix of How Big Things Get Done (Flyvbjerg & Gardner, 2023). This is the most accessible form of the data.
- The methodology and underlying figures appear across Flyvbjerg's peer-reviewed papers, most of which are free to download from his SSRN author page. The canonical overview is “What You Should Know About Megaprojects and Why” (Project Management Journal, 2014).
- Flyvbjerg's own site, bentflyvbjerg.com, links his current papers and books as they are released.
If a source claims to link “the Flyvbjerg database” as a downloadable dataset, treat it with caution: it is almost certainly a derived table of the published base rates, not the original record-level data.
How to cite
Source: bentflyvbjerg.com