Air　filters　are　one　effective　way　of　removing　indoor　fine　particulate　matter　(generally　known　as　PM2.5).　An　accurate　evaluation　of　indoor　PM2.5　filtration　load　will　provide　great　help　in　designing　efficient　air　filtration　systems.　In　order　to　make　the　existing　indoor　PM2.5　filtration　load　calculation　model　more　available　by　considering　the　availability　of　some　important　parameters,　such　as　air　infiltration　volume　through　a　unit　length　of　external　window　(ql),　particle　penetration　factor　(P),　and　deposition　rate　(k),　this　study　introduces　a　determination　method　of　those　key　design　parameters　for　various　external　window　airtightness　levels　under　infiltration　conditions.　Furthermore,　the　indoor　PM2.5　filtration　loads　were　quantified　based　on　a　longitudinal　field　measurement　from　five　unoccupied　offices　located　in　the　central　area　of　Beijing,　China.　Results　revealed　that　higher　airtightness　level　windows　can　help　to　significantly　decrease　the　indoor　PM2.5　filtration　load.　The　proposed　method　in　this　study　can　help　in　the　calculation　of　indoor　PM2.5　filtration　load　through　infiltration　and　the　design　of　indoor　air　filtration　systems.　©　2020,　Springer　Nature　Singapore　Pte　Ltd.