Alternating posture between sitting and standing with sit-stand desks may improve lower back pain (LBP). This study explored the effectiveness, acceptability and adherence of two sit-stand desk recommendations on LBP (measured via App) and secondary health and work outcomes. Fifty-six desk-based workers with LBP (age = 37.7 [SD = 10.0] years, 75 % women) were randomised to either a fixed ratio (30 min sitting:15 min standing); or a personalised ratio (self-selected or pre-emptive to discomfort). After 3 months, fixed ratio participants experienced reduced worst (-1.33 points/10) and average LBP (-0.83 points/10); personalised ratio participants experienced reduced worst LBP only (-0.69 points/10). Fixed ratio participants improved in their job-related stress, concentration and presenteeism; whereas, personalised ratio participants reduced their sitting time. Both recommendations were acceptable, while adherence was higher in the fixed ratio group. In conclusion, a 30:15 sit-stand ratio was acceptable and effective at improving LBP in the short term compared to a personalised ratio.