How do I convert a USDA rating to RHS?

Match the temperature range. USDA hardiness zone bands and RHS hardiness rating bands are both defined by temperature, so a USDA rating maps onto whichever RHS band or bands cover the same range. The match is usually approximate rather than exact, because the two systems divide the scale differently.

Do USDA and RHS mean the same thing?

No. Plant tolerates down to this zone. Plant needs at least this level of cold tolerance. They are different claims, and reading one as the other is the most common cause of a plant being bought for the wrong climate.

USDA to RHS — hardiness zone conversion table

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USDA hardiness zone is used in United States; RHS hardiness rating is used in United Kingdom. Both are anchored to temperature, so they can be lined up — but they divide the scale differently, and one band in one system often spans two in the other. Treat the table below as the nearest equivalent, not a precise translation.

They are not the same kind of claim. Plant tolerates down to this zone. Plant needs at least this level of cold tolerance. That difference matters more than the numbers: reading one as though it were the other is the single most common way a plant ends up bought for the wrong garden.

USDA to RHS conversion table

Each USDA band against the RHS band or bands covering the same temperatures
USDA	Temperature range	RHS equivalent	What it means
Zone 1	down to −45.6 °C	H7	Sub-arctic — winter lows below −45 °C
Zone 2	−45.6 °C to −40 °C	H7	Severe continental cold
Zone 3	−40 °C to −34.4 °C	H7	Very cold winters
Zone 4	−34.4 °C to −28.9 °C	H7	Cold winters, short growing season
Zone 5	−28.9 °C to −23.3 °C	H7	Cold continental winters
Zone 6	−23.3 °C to −17.8 °C	H7, H6	Moderately cold winters
Zone 7	−17.8 °C to −12.2 °C	H6, H5	Mild cold winters
Zone 8	−12.2 °C to −6.7 °C	H5, H4	Mild winters with light freezes
Zone 9	−6.7 °C to −1.1 °C	H4, H3	Light frost only
Zone 10	−1.1 °C to 4.4 °C	H3, H2	Near frost-free
Zone 11	4.4 °C to 10 °C	H2, H1c	Frost-free, warm winters
Zone 12	10 °C to 15.6 °C	H1b, H1a	Tropical
Zone 13	15.6 °C and warmer	H1a	Hot tropical

Why the conversion is approximate

A plant rated USDA zone X survives in zone X and warmer. Your zone number should be at or above the plant's rating to grow it outdoors.

RHS ratings run from H1a (greenhouse only) up to H7 (very hardy). Your local rating should be at or above the plant's rating to grow it outdoors.

Because the two systems were built for different places and different questions, a converted number tells you roughly where a plant sits on the cold scale — not whether it will thrive. Summer heat, humidity, rainfall pattern and the length of your growing season all matter, and none of them appears in either rating.

Check a rating against your own conditions

Converting the number is only half the job. Cross-check a rating against your own climate, or find your zones in all four systems by answering a short set of questions about where you garden.

See also the full band tables for USDA hardiness zone and RHS hardiness rating.