How to calculate Node Allocatable Resources for Kubernetes cluster?

As you mentioned you are using OpenShift and docs you provided are from GCP. Default requirements, parameters might be differnt due to Cloud Provider specs.

Unfortunately I am not OpenShift user, but you can find in OS documentation that:

Resources reserved for node components are based on two node settings: kube-reserved and system-reserved.

You can set these in the kubeletArguments section of the node configuration file (the /etc/origin/node/node-config.yaml file by default) using a set of = pairs (e.g., cpu=200m,memory=512Mi).

How to compute Allocated Resources?

An allocated amount of a resource is computed based on the following formula:

[Allocatable] = [Node Capacity] - [kube-reserved] - [system-reserved]

If [Allocatable] is negative, it is set to 0.

Please check below OpenShift docs: Allocating node resources, Capacity management, Cluster Limits, Resource Limits

Many factors depends on what exactly pods/images you want to use. Some images might requested 0.1 CPU but others might need 1CPU to start.

You can limit it by create a Quota, set Pod requests and limits.

Please keep in mind that you can always check current Requests/Limits in each Pod, under the Containers.containerName.Requests:

$ os describe pod <pod-name>

Or requested resources / limits on node

$ os describe node <node-name>

On the bottom of this description you should get All pods reqyest and limits

Non-terminated Pods:         (6 in total)  Namespace                  Name                                              CPU Requests  CPU Limits  Memory Requests  Memory Limits  AGE  ---------                  ----                                              ------------  ----------  ---------------  -------------  ---  default                    nginx-7cdbd8cdc9-b94r9                            100m (10%)    0 (0%)      0 (0%)           0 (0%)         6m2s  default                    nginx-7cdbd8cdc9-nlsw7                            100m (10%)    0 (0%)      0 (0%)           0 (0%)         6m2s  kube-system                fluentd-gcp-v3.2.0-lwnqn                          100m (10%)    1 (106%)    200Mi (7%)       500Mi (18%)    5h22m  kube-system                kube-proxy-gke-stc1-default-pool-094e5c74-4dzj    100m (10%)    0 (0%)      0 (0%)           0 (0%)         5h22m  kube-system                prometheus-to-sd-lbj57                            1m (0%)       3m (0%)     20Mi (0%)        20Mi (0%)      5h22m  kube-system                traefik-749d86f748-frs7q                          0 (0%)        0 (0%)      0 (0%)           0 (0%)         158mAllocated resources:  (Total limits may be over 100 percent, i.e., overcommitted.)  Resource                   Requests    Limits  --------                   --------    ------  cpu                        401m (42%)  1003m (106%)  memory                     220Mi (8%)  520Mi (19%)  ephemeral-storage          0 (0%)      0 (0%)  attachable-volumes-gce-pd  0           0

Hope it will help.