-- The WHERE clause must be evaluated for each row in the Stock table

-- in order to determine which rows qualify for the UPDATE.

-- The likelyhood of a table lock might be high.

-- So, determine which rows qualify without locking the stock table for update.

-- Capture the PK of the stock table, and the new value.

SELECT s.PrimaryKey, x.NewequivQty

INTO #Stock

FROM Stock s

LEFT OUTER JOIN Pack p

ON s.Product = p.ProductId

AND s.Pack = p.PackId

CROSS APPLY (

SELECT

OldEquivQuantity = ISNULL(s.equivQty,0),

NewequivQty = s.Qty * ISNULL(p.Factor,4.5) / 4.5

) x

WHERE x.OldEquivQuantity <> x.NewequivQty

-- Then apply the update, touching only the rows which will be updated

-- The downside? You end up reading the stock table twice.

UPDATE s SET equivQty = NewequivQty

FROM Stock s

INNER JOIN #Stock st

ON st.PrimaryKey = s.PrimaryKey