What Larger Cities Reveal About the Material Cost of Urban Life

Concrete, steel, timber and pavement accumulate in a city over decades. Counting that stock can reveal how much physical infrastructure supports its residents, but the totals do not show whether a place uses material efficiently or simply lacks enough housing and services. Comparing cities requires an account of both the material in their structures and the lives those structures support.

That problem sits behind an October 26 Nature Cities study comparing population and estimated material mass across 3,588 cities and 17,497 analytical neighborhoods: hexagons averaging 36 square kilometers, not administrative districts. 1

Mass is estimated from building volumes, paved areas and material-intensity coefficients. Buried utilities are omitted. Subway infrastructure enters city totals but not the main comparison used to estimate the population–material relationship. 1

To see what such an estimate can do, consider the difference between measuring a building's outline and weighing its contents. An overhead image can help establish how much ground a building covers. Height adds information about its volume. Neither reveals exactly how much concrete, timber or steel lies inside. Those quantities must be inferred from information about how buildings are constructed.

Material-intensity research supplies that missing bridge. A coefficient might express kilograms of a particular material per square meter of floor area. Multiplying it by the relevant area produces an estimated quantity. The coefficient must suit the building: a steel-framed structure and a masonry structure do not have interchangeable recipes. The RASMI research database, published in Scientific Data in 2024, organizes ranges by region, structural system and building function. Its authors describe uneven underlying evidence and use related observations to fill gaps. A range communicates something a single average can conceal: otherwise similar buildings can contain different quantities. 2

Improving the map therefore requires more than sharper satellite pictures. Better construction records can change the estimated material behind the same visible shape. An uncertainty in the conversion factor travels into the total, even when the outline of the building is measured accurately.

The result is an inventory assembled from measurements and assumptions. A map can locate a likely concentration of material without becoming a survey of every foundation and wall. This is a useful compromise at continental scale, provided its resolution is respected. A city engineer deciding whether a particular structure can carry another floor would still need information about that structure, its condition and its loads.

After accounting for average differences among countries, the study reports a population exponent of 0.90 across cities; accounting for average differences among cities gives 0.75 within cities. Both indicate less mass per person at larger populations. Sharing is a proposed explanation, not a measured causal contribution. 1

An exponent describes how quantities change together. One would mean proportionality: twice the people, twice the material. Here the fitted mass comparisons at twice the population are approximately 1.87 and 1.68, respectively. Neither is a forecast for one growing city. The similar-area neighborhood cells also make their population a measure of density.

Sharing is physically plausible because infrastructure can serve more than one household. A roof shelters several apartments; an entrance provides access to multiple homes. If twenty additional homes can use an existing street, the same pavement serves more households. If serving them requires a wider road and rebuilt junctions, the material calculation changes. An additional apartment may also require stronger supports. The relevant question is how much extra material delivers an additional unit of useful service, once those requirements are included. Counting residents provides a starting denominator, not a complete engineering specification.

A separate Nature Cities paper published the same day helps explain why the relationship cannot simply be turned into a growth rule. Ulysse Marquis and Marc Barthelemy argue that comparisons among cities combine different histories, constraints and institutions. A pattern across places can differ from the path followed by any one place. Their analysis concerns urban scaling more broadly, including the relationship between urban area and population; it is not an independent replication of the material estimates. 3

A hypothetical comparison makes the problem tangible. One established city may have filled vacant buildings over decades. Another may have built new districts before residents arrived. Comparing their present populations and infrastructure would mix those histories. Neither city is necessarily a preview of the other's future. A local decision therefore needs evidence about what will actually be added, reused or retired as population changes.

Time creates another accounting problem. Material stock is the material embodied in structures at a given time; material flow is what enters or leaves during a period. A building standing for decades remains part of the stock each year without being rebuilt each year. Replacing a deteriorated road surface creates demand for new material even if the total pavement area stays constant. A place can maintain a stable inventory while continually consuming materials to repair it. Any claim about future extraction or waste must account for that turnover.

Nor does a tonne of material translate directly into a tonne of climate pollution. The Royal Institution of Chartered Surveyors' whole-life-carbon standard combines material quantities with emissions factors and includes stages from production and construction through use and end of life. Manufacturing methods, transport and replacement requirements can change the result. A lighter component that needs repeated replacement could compare differently with a heavier, longer-lived alternative than their initial weights suggest. That example requires a lifecycle calculation, not an assumption that either option wins. 4

Money is another separate account: quantities need prices, labor, maintenance and timing before they become a financial comparison. And minimizing tonnage cannot, by itself, define a successful city. A neighborhood lacking adequate housing or sanitation might look materially sparse for reasons no planner should celebrate.

Research on decent living standards offers a complementary starting point. A 2025 Nature Sustainability analysis estimates material requirements from services needed to meet basic human needs, including shelter, water, health care and mobility. It distinguishes provision of those services from demand beyond them. That framework directs attention toward what infrastructure enables, rather than treating accumulation or reduction as the objective. 5

For city leaders, the promising use of a material map is to identify questions worth investigating. Where could existing capacity serve additional residents? Where would more people require rebuilding? Which places have little material because they share it effectively, and which because essential services are missing? Answering those questions would connect the inventory to transport, housing, maintenance and access records.

A street's value lies in the homes it connects and the journeys it supports. A better material account would establish how many people it can serve reliably, what changes would be needed, and how long those changes would last. The measure of progress would be useful, durable services delivered with fewer unnecessary additions to the city's physical burden.

Sources

• Huang and Lu, Nested economies of scale in global city mass, Nature Cities, October 26, 2026

• Fishman and colleagues, RASMI: Global ranges of building material intensities differentiated by region, structure, and function, Scientific Data, April 23, 2024

• Marquis and Barthelemy, Urban scaling and the dynamics of individual cities, Nature Cities, October 26, 2026

• Royal Institution of Chartered Surveyors, Whole life carbon assessment for the built environment, second edition, version 3, August 2024

• Streeck and colleagues, Small increases in material stocks to achieve decent living standards globally, Nature Sustainability, November 5, 2025

• Huang and Lu, associated data and code record, Figshare