Mounting a ConfigMap as a volume in Kubernetes: how do I calculate the value of defaultMode?

Defining the defaultMode in a Kubernetes volume field within a deployment element can become quite tricky.

It expects three decimals, corresponding to the binary UNIX permissions.

As an example, to mount the ConfigMap with permissions r------, you'd need to specify 256.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: foo
  namespace: foo
spec:
  replicas: 1
  selector:
    matchLabels:
      app: foo
  template:
    metadata:
      labels:
        app: foo
    spec:
      containers:
        - image: php-fpm:latest
          volumeMounts:
            - name: phpini
              mountPath: /usr/local/etc/php/conf.d/99-settings.ini
              readOnly: true
              subPath: 99-settings.ini
      volumes:
        - configMap:
            defaultMode: 256
            name: phpini-configmap
            optional: false
          name: phpini
---
apiVersion: v1
kind: ConfigMap
metadata:
  labels:
    app: foo
  namespace: foo
  name: phpini-configmap
data:
  99-settings.ini: |
    ; Enable Zend OPcache extension module
    zend_extension = opcache

Use the following table:

A more direct way to do this is to use a base8 to base10 converter like this one

Help from Kubernetes docs

defaultMode is optional: mode bits used to set permissions on created files by default. Must be an octal value between 0000 and 0777 or a decimal value between 0 and 511. YAML accepts both octal and decimal values, JSON requires decimal values for mode bits. Defaults to 0644. Directories within the path are not affected by this setting. This might be in conflict with other options that affect the file mode, like fsGroup, and the result can be other mode bits set.