Reference library · TV budgeting
Spot Length Coefficient
The Spot Length Coefficient (SLC) is the multiplier applied to a non-30-second spot's delivered GRPs to convert them into 30-second-equivalent GRPs for costing purposes. It is a published or negotiated market convention, not simply the duration ratio, and lets a planner price a 15" buy on the same CPP scale as a 30" benchmark.
Formula
Worked example
A 15" spot runs 25 times with an average target-group rating of 8.0, giving raw 15" GRPs of 25 x 8.0 = 200 GRPs. The channel's published SLC for 15" spots this year is 0.65, so the 30"-equivalent delivery is 200 x 0.65 = 130 GRPs. At the market's standard 30" CPP of €120, the booking cost is 130 x €120 = €15,600. The plan then reconciles against invoiced GRPs using the same 130-GRP equivalent figure, not the raw 200.
How it is used
Media owners and sales houses publish an SLC table per market, channel and sometimes daypart, and it gets revised at each rate-card update, so planners pull the current figure rather than reuse last year's. It is applied to the GRP delivery before CPP is calculated, so cost comparisons across spot lengths stay on a common 30" basis. The mistake most planners make is assuming SLC equals the duration ratio (15/30 = 0.5) and hand-calculating it instead of checking the negotiated table, which usually sits higher (0.6-0.7) because shorter spots carry a per-second premium.
The common mistake
Use the market's published SLC, not the 15/30 duration ratio, or the 30"-equivalent GRPs and resulting cost will be understated.