Methodology

Background

Understanding affordability in cities requires looking beyond prices alone. Two long-standing concepts are commonly used to describe economic pressure on households: Engel’s Index and the 30% housing rule.

Each captures a different dimension of living costs. The Urban Stress Index (USI) attempts to cover both dimensions under single index.

Engel’s Index

Engel’s Index originates from Engel’s Law, which observes that as income increases, the proportion of expenditure spent on basic necessities — especially food — declines.

A higher Engel’s Index indicates that a larger share of income is required for essential consumption.
This suggests greater vulnerability to price shocks and limited discretionary capacity.

In its original formulation, Engel’s Index measures the share of food expenditure relative to total household expenditure. In the context of USI, an adapted approach is used: essential food cost is expressed as a share of gross income.

For decades, Engel’s Index has been used as a proxy for living standards, household stress,and levels of economic development. However, it was developed in contexts where housing costs were relatively stable and did not dominate household budgets.This income-based proxy is intended to reflect structural affordability pressure rather than consumption allocation patterns. While not identical to classical Engel’s Index, it preserves the core insight that essential cost burden signals economic stress.

The 30% Housing Rule

The 30% housing rule is a widely used benchmark in housing policy and urban planning.

A household spending more than 30% of its gross income on housing is considered housing-cost burdened.

Once housing costs exceed a certain share of income, trade-offs emerge in food, healthcare, mobility, and savings. Despite its simplicity, the 30% rule remains a practical signal of housing stress across cities and countries.

Housing as a Dominant Fixed Cost in Modern Cities

As societies urbanise and incomes rise, the structure of household expenditure changes. In contemporary cities, food and basic consumption often scale gradually with income. Housing costs, by contrast, are rigid, location-dependent, and increasingly detached from local wage growth.

Neither Engel’s Index nor the 30% rule alone fully captures this reality:

Engel’s Index underweights housing stress.
The 30% rule ignores essential non-housing consumption.

Rationale for the Urban Stress Index (USI)

The Urban Stress Index (USI) combines these two perspectives. By summing an Engel-style essential food cost share and a housing burden component, USI reflects the combined pressure of essential living needs and fixed housing costs relative to income.

The aim is not to predict individual hardship, but to provide a comparable, structural view of cost-of-living pressure across cities.

Index Definition

The Urban Stress Index (USI) is defined as:

USI = Essential Food Cost Share + Housing Burden

Both components are expressed as percentages of gross income.

Essential Food Cost Share

This component follows the logic of Engel’s Index, representing the proportion of income required to cover essential food cost for a single person.

Rather than attempting to model full food cost budgets, the index uses a simplified proxy based on the following:

prices from inexpensive restaurants
a weekly ratio of 1 dine-out meal with 5 home-cooked meals
breakfast is assumed to count as 0.5 proportion of food

The purpose is to capture baseline living pressure, not lifestyle-dependent spending.

Housing Burden

The housing component represents the share of gross income required to rent a typical market-rate one-bedroom unit in the city.

Housing is treated as a fixed cost
Housing is location-dependent
Housing is largely non-negotiable in the short term

This follows the logic of the 30% housing rule, while allowing the burden to scale continuously beyond the threshold.

Cost and Income Assumptions

Household type: single adult
Housing type: market-rate one-bedroom rental
Income metric: city-level average or median gross income
All values reflect pre-tax income and typical market conditions

The index does not account for:

household size variation
shared accommodation
home ownership or subsidised housing
individual tax situations or government benefits

USI is therefore best interpreted as a structural indicator, not a personalised affordability score.

Classification Thresholds

< 30 — Comfortable
30–40 — Stretched
40–50 — High burden
50–60 — Severe burden
60–70 — Unaffordable
70–100 — Extreme
> 100 — Critical (costs exceed income)

The >100 category represents a structural breakdown where basic living costs — housing and essential food — exceed total income. In such cases, households must rely on trade-offs such as shared housing, reduced consumption, external support, or debt.

These thresholds are interpretive and designed to distinguish escalating levels of cost pressure, from manageable conditions to systemic imbalance.

Interpretation Guidelines

A higher USI indicates greater structural pressure on everyday urban living.
Cities with similar USI values may still differ in quality of life, public services, or social safety nets.
The index is most useful for relative comparison across cities and regions.

USI should be read as a signal, not a verdict.

Limitations

City-level averages may mask intra-city inequality.
Cost data represent a snapshot in time and may lag rapid market changes.
Informal housing arrangements and cultural consumption differences are not captured.
The index simplifies complex household decisions for the sake of comparability.

These limitations are accepted trade-offs to preserve transparency and consistency.

Versioning and Scope

This methodology applies to USI v1.

Future versions may refine consumption proxies, adjust thresholds, or introduce sensitivity analysis. Methodological stability is prioritised to ensure comparability over time.

USI is not about how cheaply someone can live in a city, but about how much pressure a city places on living by default.