Free intraclass correlation coefficient (ICC) calculator: paste your data matrix (rows = subjects, columns = raters or measurements), answer two plain-language questions and get all six McGraw and Wong (1996) forms — ICC(1,1), ICC(1,k), ICC(2,1), ICC(2,k), ICC(3,1) and ICC(3,k) — with 95% CIs, the intermediate ANOVA table, the Koo and Li (2016) interpretation and an APA 7 or Vancouver sentence ready to paste into your manuscript.
Worked example: 2 physiotherapists measure knee flexion in 15 patients with a goniometer; absolute-agreement ICC(2,1) = .896, 95% CI [.723, .964], F(14, 14) = 19.51, p < .001: good reliability according to Koo and Li.
Which ICC model should I choose? If the same raters score every subject and are a sample of possible raters, ICC(2, absolute agreement); if they are the only raters of interest, ICC(3, consistency); if each subject has different raters, ICC(1). What value is acceptable? Below .50 poor reliability, .50 to .75 moderate, .75 to .90 good and above .90 excellent (Koo and Li, 2016), always interpreting the confidence interval.
Intraclass correlation coefficient for inter-rater or test-retest reliability: all six McGraw & Wong forms with 95% CIs and the APA 7 sentence.
If you use it in your thesis or in a paper, cite it so that readers of your Methods section know how the calculation was done.
Ciudad-Fernández, V. (2026). Intraclass correlation coefficient (ICC) calculator [Web application]. Ciudad Science. Retrieved October 14, 2026, from https://analisisdedatospsicologia.com/en/tools/icc-calculator
For your Methods section: Reliability was estimated with the intraclass correlation coefficient (Shrout & Fleiss, 1979; McGraw & Wong, 1996), with the model selected and interpreted following Koo and Li (2016), using the Ciudad Science intraclass correlation coefficient (ICC) calculator (Ciudad-Fernández, 2026).
@misc{ciudadfernandez2026icc,
author = {Ciudad-Fern{\'a}ndez, V{\'i}ctor},
title = {{Intraclass correlation coefficient (ICC) calculator}},
year = {2026},
publisher = {Ciudad Science},
howpublished = {Web application},
url = {https://analisisdedatospsicologia.com/en/tools/icc-calculator},
urldate = {2026-10-14},
note = {Accessed 2026-10-14},
}