# What is Maximum Tolerable Downtime (MTD)?

> Maximum tolerable downtime is the absolute ceiling on how long a process or service can be unavailable before the impact becomes unacceptable — the outer bound that RTOs must stay within.

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## Content

Where RTO is the planned target for restoring service, MTD is the hard limit beyond which damage — regulatory, contractual, financial, safety — becomes intolerable. Business impact analysis establishes MTD per process; continuity planning then designs recovery to land well inside it, since real recovery tends to overrun its plan.

### Key points

- **Per-process setting:** The same system serving two processes inherits the stricter of their MTDs; derive it from business consequence, not infrastructure convenience.
- **Margin under the limit:** RTO should sit comfortably below MTD — plans that target exactly the maximum have no room for the delays real incidents bring.
- **Important limitation:** MTD is an estimate of tolerance, not a measured threshold. It is set before the incident by analysis and judgment; the real point of failure may arrive sooner under conditions the BIA did not model.

### Related terms

[Recovery time objective (RTO)](<https://yellowcube.eu/glossary/recovery-time-objective/>) · [Recovery point objective (RPO)](<https://yellowcube.eu/glossary/recovery-point-objective/>) · [Business impact analysis (BIA)](<https://yellowcube.eu/glossary/business-impact-analysis/>) · [Business continuity](<https://yellowcube.eu/glossary/business-continuity/>) · [Disaster recovery](<https://yellowcube.eu/glossary/disaster-recovery/>)

### Sources

[NIST SP 800-34 Rev. 1, Contingency Planning Guide for Federal Information Systems](https://csrc.nist.gov/pubs/sp/800/34/r1/upd1/final) · [ISO 22301, Business Continuity Management Systems](https://www.iso.org/standard/75106.html)

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