# DADA2, truncation lengths and features number

**URL:** https://forum.qiime2.org/t/dada2-truncation-lengths-and-features-number/1940
**Category:** User Support
**Tags:** dada2
**Created:** [November 21, 2017, 2:50pm UTC](https://forum.qiime2.org/t/dada2-truncation-lengths-and-features-number/1940 "2017-11-21T14:50:23Z")
**Posts on this page:** 1
**Showing post:** 9

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### Author: ![benjjneb](https://forum.qiime2.org/user_avatar/forum.qiime2.org/benjjneb/32/1602_2.png) [@benjjneb](https://forum.qiime2.org/u/benjjneb)
#### Post date: [November 22, 2017, 2:32pm UTC](https://forum.qiime2.org/t/dada2-truncation-lengths-and-features-number/1940/9 "2017-11-22T14:32:58Z")

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> [@mchialva](#):
>
> Looking at profiles I would truncate reverse reads at 220-240 but I’m not sure what is the minimum acceptable overlap between reads. 50-60 bp might make sense?

Minimum overlap is 20 nts for merging, but you need to factor in some biological length variation in the amplicon size as well, so I'd say at least 30nts of overlap for V3/V4.

> [@mchialva](#):
>
> A different left reads and right reads truncation value would be acceptable?

Not just acceptable, usually the right choice. Here the reverse reads are clearly worse quality, so should be truncated earlier than the forward reads. Perhaps `trunc-len-f 280` and `trunc-len-r 220`? That gives 500 total nts after truncation, so still safely overlapping, and gets rid of the worst quality region.

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