Methodology
How the household inflation estimate works
This is an indicative household estimate of your own inflation rate, also called a personal inflation rate. It combines reviewed Stats NZ price movements with transparent assumptions about the share of spending represented by each household answer. It is not an official personal CPI.
What the calculator is estimating
Headline CPI uses a national expenditure basket. The calculator starts with that basket, adjusts its component weights using eight quick household answers, normalises the adjusted basket back to 100%, and applies the current annual price movement for each component.
The Stats NZ price movements themselves are never changed. The calculator only changes the estimated importance of each component to your household.
Which figures are shown, and which drive the maths
Each component carries two annual-change figures. The figure displayed on the site is the annual percentage change as published by Stats NZ, so every number you read here matches the official release. The figure the calculation engine uses is derived from Stats NZ's percentage-point contributions and scaled so the full basket reproduces the official headline CPI exactly. The two agree closely; they can differ by a fraction of a percentage point on small components because published figures are rounded.
What the eight questions change
Household size
Applies modest adjustments to food, energy, housing, education and recreation.
Housing
Separates rent from owner-occupied housing, maintenance, rates and home insurance.
Transport
Separates vehicle ownership, fuel, servicing, insurance and public transport.
Food at home
Adjusts produce, meat, seafood and other grocery weights by dietary pattern.
Meals away from home
Adjusts restaurant and takeaway spending separately from dietary preference.
Energy sources
Includes or excludes electricity, gas and solid fuels independently.
Energy use
Scales the selected household energy components for low, typical or high use.
Standout costs
Optionally increases up to two areas that take a larger-than-usual budget share.
How removed spending is handled
Some answers set components to zero. For example, a no-car household receives no petrol, vehicle-purchase, servicing or vehicle-insurance weight. The remaining component weights are then increased proportionally so the basket still totals 100%.
This does not claim that every dollar saved on a car is spent proportionally across everything else. It is a neutral quick-estimate assumption. Entering actual category expenditure in a future detailed mode would replace this redistribution with the household's own weights.
What sits inside “Other spending”
The calculator tracks the CPI components that household answers can move, such as rent, fuel, groceries and energy. Everything else in the CPI basket is gathered into a single “Other household spending” component so the basket still totals 100% and still reproduces the headline rate. It covers the expenditure the questions do not distinguish between, and it is roughly a sixth of the basket.
Its weight is calculated as the remainder once every mapped component is accounted for, and its price movement is set so that the mapped components plus this remainder reproduce the official headline CPI. Because no household answer changes it, it applies equally to every profile and quietly anchors the estimate to the national figure.
Current assumption register
The tables below publish the exact component-weight multipliers used by model version 0.4-expanded-questionnaire. A multiplier of 1.00× leaves the national weight unchanged; 0 excludes the component. Multiple applicable rules are multiplied together before normalisation.
Household-size coefficients
The two-adult, no-child household is the reference size. For each person above or below two, the model changes selected weights by the following percentage coefficients. Child coefficients are applied in addition.
| Component | Per person from reference | Additional per child |
|---|---|---|
| Other groceries | 6.0% | 4.0% |
| Fruit and vegetables | 5.0% | — |
| Meat and poultry | 5.0% | — |
| Seafood | 4.0% | — |
| Electricity | 4.5% | — |
| Gas | 3.0% | — |
| Rent | 2.5% | — |
| Owner-occupied housing | 2.5% | — |
| Education and childcare | 4.0% | 55.0% |
| Recreation and travel | 3.0% | 8.0% |
Housing rules
| Answer | Component-weight adjustment |
|---|---|
| Renting | Rent: 1.80×; Owner-occupied housing: excluded; Property maintenance: 0.15×; Council rates: excluded; Home insurance: excluded; Contents insurance: 0.85×; Household goods: 0.90× |
| Homeowner with a mortgage | Rent: excluded; Owner-occupied housing: 1.55×; Property maintenance: 1.25×; Council rates: 1.25×; Home insurance: 1.25×; Contents insurance: 1.10×; Household goods: 1.10× |
| Homeowner without a mortgage | Rent: excluded; Owner-occupied housing: 1.55×; Property maintenance: 1.25×; Council rates: 1.25×; Home insurance: 1.25×; Contents insurance: 1.10×; Household goods: 1.15× |
| Boarding or bundled housing costs | Rent: 0.75×; Owner-occupied housing: excluded; Property maintenance: 0.05×; Council rates: excluded; Home insurance: excluded; Contents insurance: 0.30×; Electricity: 0.55×; Gas: 0.35×; Wood and other solid fuels: 0.35×; Household goods: 0.60× |
| General housing pattern | No adjustment; the national component weight is retained. |
Transport rules
| Answer | Component-weight adjustment |
|---|---|
| No household car | Vehicle purchases: excluded; Vehicle parts: excluded; Petrol: excluded; Other vehicle fuels: excluded; Vehicle servicing: excluded; Other private transport: 0.05×; Vehicle insurance: excluded; Public transport: 2.30× |
| One car used occasionally | Vehicle purchases: 0.55×; Vehicle parts: 0.55×; Petrol: 0.35×; Other vehicle fuels: 0.35×; Vehicle servicing: 0.60×; Other private transport: 0.75×; Vehicle insurance: 0.85×; Public transport: 1.30× |
| One car used regularly | No adjustment; the national component weight is retained. |
| Two or more cars | Vehicle purchases: 1.55×; Vehicle parts: 1.45×; Petrol: 1.65×; Other vehicle fuels: 1.65×; Vehicle servicing: 1.50×; Other private transport: 1.40×; Vehicle insurance: 1.55×; Public transport: 0.65× |
Diet and meals-away-from-home rules
| Answer | Component-weight adjustment |
|---|---|
| Typical mixed diet | No adjustment; the national component weight is retained. |
| Meat-heavy diet | Meat and poultry: 1.75×; Fruit and vegetables: 0.82×; Seafood: 0.75× |
| Pescatarian diet | Meat and poultry: 0.06×; Seafood: 2.20×; Fruit and vegetables: 1.42×; Other groceries: 1.10× |
| Vegetarian or vegan diet | Meat and poultry: 0.03×; Seafood: 0.03×; Fruit and vegetables: 1.65×; Other groceries: 1.18× |
| Answer | Component-weight adjustment |
|---|---|
| Restaurants and takeaways less often than most | Restaurant meals and takeaways: 0.45×; Other groceries: 1.08× |
| Typical restaurant and takeaway spending | No adjustment; the national component weight is retained. |
| Restaurants and takeaways more often than most | Restaurant meals and takeaways: 2.10×; Other groceries: 0.82× |
Household energy rules
| Answer | Component-weight adjustment |
|---|---|
| Electricity only | Electricity: 1.12×; Gas: excluded; Wood and other solid fuels: excluded |
| Electricity and gas | Electricity: 1.00×; Gas: 3.10×; Wood and other solid fuels: excluded |
| Electricity and wood or other solid fuel | Electricity: 0.95×; Gas: excluded; Wood and other solid fuels: 5.50× |
| Several household energy sources | Electricity: 1.00×; Gas: 2.20×; Wood and other solid fuels: 3.50× |
| Answer | Component-weight adjustment |
|---|---|
| Lower-than-typical household energy use | Electricity: 0.62×; Gas: 0.62×; Wood and other solid fuels: 0.62× |
| Typical household energy use | No adjustment; the national component weight is retained. |
| Higher-than-typical household energy use | Electricity: 1.65×; Gas: 1.65×; Wood and other solid fuels: 1.65× |
Optional higher-spending rules
| Answer | Component-weight adjustment |
|---|---|
| Health and medical costs | Health and medical costs: 1.55× |
| Education or childcare | Education and childcare: 1.60× |
| Travel and recreation | Recreation and travel: 1.55× |
| Council rates | Council rates: 1.45× |
| Insurance premiums | Home insurance: 1.40×; Contents insurance: 1.40×; Life insurance: 1.40×; Vehicle insurance: 1.25× |
| Technology and communications | Technology and communications: 1.55× |
Housing and mortgage interest
CPI includes actual rent, new owner-occupied housing, maintenance, rates, household energy and relevant insurance components. It does not include mortgage interest. Mortgage holders and outright owners therefore receive similar ownership-cost treatment in this CPI-based estimate.
Changes in an individual mortgage rate or repayment can have a major effect on cashflow, but they are outside the current calculation and should not be inferred from the result.
Relationship to the Household living-costs price indexes (HLPIs)
Stats NZ separately publishes the Household living-costs price indexes, which measure inflation as experienced by broad household groups — such as beneficiary, Māori, superannuitant, and income- and expenditure-quintile households. Unlike CPI, the HLPIs include interest payments, which is why a mortgaged household's living costs can rise much faster than headline CPI when interest rates increase.
This calculator is CPI-based, so it answers a different question: how the CPI basket's price movements combine for a household with your spending pattern. It does not replicate or replace the HLPIs.
Important limitations
- The result is a broad estimate, and it is not an official statistic.
- The profile multipliers are modelling assumptions rather than Stats NZ measures.
- The exact multiplier values are provisional and may be recalibrated in later model versions.
- It does not know the exact products, brands, shops, quantities or prices used by the household.
- It does not measure lifestyle changes, savings, mortgage principal or every cashflow change.
- Small differences should not be interpreted as statistically significant.
- It is a general reference and does not constitute personal financial advice.
Versioning and review
Each quarterly release records the model version used to generate its data. Automated checks confirm that the baseline basket totals 100%, reproduces the official headline CPI, excludes incompatible household components, remains normalised after every profile adjustment, and that no single component can grow to an implausible share of the basket through stacked multipliers.
Privacy
The calculator runs in the browser. Answers and optional dollar amounts are not sent to a database.