Start by defining what would likely have happened without the borrow. Would the person have bought the item new, borrowed from a neighbor, hired, or simply gone without? Documenting these options with simple survey prompts prevents over-claiming. Use conservative defaults where data is thin, and adjust by category. For example, power tools may replace purchases more often than party gear. Keeping assumptions explicit makes the math defensible, comparable across libraries, and easier to revise as better evidence arrives.
Every share builds extra utility into an item’s life, but only if it remains functional and safe. Count uses, downtime, repairs, and early retirements. Maintenance logs and routine inspections help estimate remaining life, unlocking better avoided-purchase calculations. When an asset’s performance drifts, factor the change rather than ignoring it. Simple rules-of-thumb per category, refined by real repair data, can show how a sharpened blade, new battery, or resewn seam prevents premature replacement, strengthening both carbon and waste reduction estimates.
Convert activities into impact using reputable sources: UK Government greenhouse gas conversion factors for energy and transport, product footprint studies for embodied emissions, and waste treatment datasets for end-of-life outcomes. Document which source you used, the year, and any adjustments. Show formulas openly so anyone can audit the approach. If multiple credible values exist, present ranges and explain why you chose the conservative end. Clear references, simple spreadsheets, and clean metadata keep trust high and make collaboration with councils, funders, and researchers straightforward.
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