{"id":126,"date":"2026-11-10T16:40:00","date_gmt":"2026-11-10T16:40:00","guid":{"rendered":"https:\/\/tothepowerof2.com\/blog\/?p=126"},"modified":"2026-11-11T03:37:52","modified_gmt":"2026-11-11T03:37:52","slug":"estimating-upload-chunk-sizes-with-powers-of-two","status":"publish","type":"post","link":"https:\/\/tothepowerof2.com\/blog\/estimating-upload-chunk-sizes-with-powers-of-two\/","title":{"rendered":"Estimating Upload Chunk Sizes With Powers of Two for Faster Transfers"},"content":{"rendered":"<p><strong>TL;DR \/ Summary:<\/strong> Multipart uploads work best when chunk byte counts are powers of two aligned with buffer sizes. Calculate 2^n bytes for candidate chunk sizes on <a href=\"https:\/\/tothepowerof2.com\/\">tothepowerof2.com<\/a>, then test one step up or down if retries spike.<\/p>\n<h2>Why APIs secretly love 2^n bytes<\/h2>\n<p>S3-compatible storage, resumable HTTP uploads, and CDN ingest pipelines read chunks into ring buffers sized at 4 KiB, 64 KiB, or 1 MiB internally. When your part size is already a power of two, client libraries avoid partial buffer flushes at the end of each read.<\/p>\n<p>That does not mean decimal megabytes fail\u2014it means starting at 2^n candidates reduces off-by-one friction when docs say \u201cminimum part size 5 MiB except when\u2026\u201d and you are converting units mentally.<\/p>\n<h2>Building a candidate ladder<\/h2>\n<p>Try this ascending list for home broadband uploads:<\/p>\n<ul>\n<li><strong>2^18 bytes<\/strong> (256 KiB) \u2014 safe on memory-tight mobile browsers<\/li>\n<li><strong>2^20 bytes<\/strong> (1 MiB) \u2014 default sweet spot for many SDK examples<\/li>\n<li><strong>2^22 bytes<\/strong> (4 MiB) \u2014 better on fiber if latency is low<\/li>\n<li><strong>2^24 bytes<\/strong> (16 MiB) \u2014 watch RAM on low-end laptops<\/li>\n<\/ul>\n<p>Compute each with the homepage tool so your config files use exact integers, not rounded decimals that truncate in JSON.<\/p>\n<h2>Mapping chunk size to link speed<\/h2>\n<p>Rough rule: pick a chunk you can upload in at least one RTT window without saturating retry buffers. On 20 Mbps uplink, 1 MiB parts take under a second\u2014fine. On 1 Mbps rural uplink, 4 MiB parts increase pain when a failure forces retransmit of the whole part.<\/p>\n<p>Measure empirically. Log three trials per size; powers of two give you discrete steps for A\/B tests without arbitrary 3.7 MiB guesses.<\/p>\n<h2>Parallelism and 2^n part counts<\/h2>\n<p>Some clients cap concurrent parts at eight (2^3). If file size divided by chunk size yields a prime number of parts, you may underutilize parallelism. Nudge chunk size one power down until part count divides evenly across workers\u2014or accept a slightly larger final part.<\/p>\n<h2>Checksum and crypto block alignment<\/h2>\n<p>Hashing libraries process 64-byte or 512-bit blocks internally. Chunk boundaries rarely need to match hash blocks, but encryption modes with chained IVs care about consistent part ordering. Document which 2^n size you standardized so teammates do not mix configs across environments.<\/p>\n<h2>When smaller beats bigger<\/h2>\n<p>Mobile Safari kills background tabs; huge chunks lose progress when users switch apps. Drop to 256 KiB or 512 KiB (2^18\u20132^19) even if desktop fiber tolerates 16 MiB. User experience beats theoretical throughput.<\/p>\n<h2>Spreadsheet column for DevOps<\/h2>\n<p>Keep a team wiki table: column A = exponent n, column B = byte count from the calculator, column C = observed p95 upload time. New hires inherit evidence instead of folklore.<\/p>\n<h2>Next deploy window<\/h2>\n<p>Before changing production multipart settings, recompute your three favorite 2^n sizes on <a href=\"https:\/\/tothepowerof2.com\/\">the 2^n calculator<\/a>, run a staging upload of a gigabyte test file, and ship the winner with metrics attached.<\/p>\n<h2>Resumable uploads after disconnect<\/h2>\n<p>Smaller 2^n chunks reduce bytes re-sent after Wi-Fi drops on trains. Larger chunks win on stable fiber. Mobile-first apps should default 2^18 or 2^19 unless telemetry proves otherwise.<\/p>\n<h2>Server-side minimum part rules<\/h2>\n<p>Cloud APIs publish minimum and maximum part sizes\u2014often 5 MiB minimum except last part. When minimum is not a power of two, choose the smallest 2^n still above the floor, or use exact documented minimum if mandated. Never assume 1 MiB works because it worked on another vendor.<\/p>\n<h2>Cost accounting<\/h2>\n<p>Some providers bill per PUT request. Many small parts inflate request charges; fewer large parts inflate retry cost. Model both: requests \u00d7 price + expected retry bandwidth. Powers of two simplify spreadsheet scenarios.<\/p>\n<h2>Testing checklist before production<\/h2>\n<p>Upload 10 GB test object with three chunk sizes, record p50 and p95 duration, error rate, and CPU on client. Pick winner, document exponent n in infrastructure repo README, link calculator for future interns.<\/p>\n<h2>Browser File API limits<\/h2>\n<p>Very old browsers cap blob slice sizes differently. If audience includes legacy enterprise browsers, test 2^20 vs 2^21 slices in QA matrix. Mobile WebKit tolerates large slices until memory pressure kills tab\u2014telemetry beats assumptions.<\/p>\n<p>Progress bars feel smoother with more parts; backend costs rise. Product managers pick UX vs infra tradeoff; engineers supply 2^n options table with measured costs.<\/p>\n<h2>Edge CDN ingest quirks<\/h2>\n<p>Some CDNs re-chunk upstream regardless of client part size. Your 2^n optimization at origin still helps first mile; last mile may re-split. Trace one upload with packet capture before declaring victory in postmortem.<\/p>\n<h2>Multipart abort cleanup<\/h2>\n<p>Failed uploads leave orphan parts billed until lifecycle rules abort incomplete multipart after N days. Smaller 2^n parts mean more orphan objects if clients crash\u2014monitor incomplete upload metrics in bucket dashboard alongside chunk size experiments.<\/p>\n<p>Document chosen chunk size in runbook so on-call engineers know expected part count when purging stuck uploads after incident.<\/p>\n<p>Document the winning exponent in your client SDK config comment block so the next upgrade does not silently revert to vendor default part sizes that ignore your load testing.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>TL;DR \/ Summary: Multipart uploads work best when chunk byte counts are powers of two aligned with buffer sizes. Calculate 2^n bytes for candidate chunk sizes on tothepowerof2.com, then test one step up or down if retries spike. Why APIs secretly love 2^n bytes S3-compatible storage, resumable HTTP uploads, and CDN ingest pipelines read chunks &#8230; <a title=\"Estimating Upload Chunk Sizes With Powers of Two for Faster Transfers\" class=\"read-more\" href=\"https:\/\/tothepowerof2.com\/blog\/estimating-upload-chunk-sizes-with-powers-of-two\/\" aria-label=\"Read more about Estimating Upload Chunk Sizes With Powers of Two for Faster Transfers\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[],"class_list":["post-126","post","type-post","status-publish","format-standard","hentry","category-math-practice-examples"],"_links":{"self":[{"href":"https:\/\/tothepowerof2.com\/blog\/wp-json\/wp\/v2\/posts\/126","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/tothepowerof2.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/tothepowerof2.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/tothepowerof2.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/tothepowerof2.com\/blog\/wp-json\/wp\/v2\/comments?post=126"}],"version-history":[{"count":1,"href":"https:\/\/tothepowerof2.com\/blog\/wp-json\/wp\/v2\/posts\/126\/revisions"}],"predecessor-version":[{"id":127,"href":"https:\/\/tothepowerof2.com\/blog\/wp-json\/wp\/v2\/posts\/126\/revisions\/127"}],"wp:attachment":[{"href":"https:\/\/tothepowerof2.com\/blog\/wp-json\/wp\/v2\/media?parent=126"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/tothepowerof2.com\/blog\/wp-json\/wp\/v2\/categories?post=126"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/tothepowerof2.com\/blog\/wp-json\/wp\/v2\/tags?post=126"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}